diff --git a/NEWS b/NEWS
index c780da4c0cb..c038a5bb11c 100644
--- a/NEWS
+++ b/NEWS
@@ -45,6 +45,8 @@ PHP                                                                        NEWS
   . Implemented "Improve callbacks in ext/dom and ext/xsl" RFC. (nielsdos)
   . Added DOMXPath::quote() static method. (divinity76)
   . Implemented opt-in ext/dom spec compliance RFC. (nielsdos)
+  . Fixed bug #79701 (getElementById does not correctly work with duplicate
+    definitions). (nielsdos)
 
 - Fileinfo:
   . Update to libmagic 5.45. (nielsdos)
diff --git a/ext/dom/attr.c b/ext/dom/attr.c
index 9cefed6b747..99686acb61f 100644
--- a/ext/dom/attr.c
+++ b/ext/dom/attr.c
@@ -25,6 +25,7 @@
 
 #include "php_dom.h"
 #include "dom_properties.h"
+#include "internal_helpers.h"
 
 /*
 * class DOMAttr extends DOMNode
@@ -106,6 +107,16 @@ zend_result dom_attr_specified_read(dom_object *obj, zval *retval)
 
 /* }}} */
 
+void dom_attr_value_will_change(dom_object *obj, xmlAttrPtr attrp)
+{
+	if (attrp->atype == XML_ATTRIBUTE_ID) {
+		xmlRemoveID(attrp->doc, attrp);
+		attrp->atype = XML_ATTRIBUTE_ID;
+	}
+
+	dom_mark_ids_modified(obj->document);
+}
+
 /* {{{ value	string
 readonly=no
 URL: http://www.w3.org/TR/2003/WD-DOM-Level-3-Core-20030226/DOM3-Core.html#ID-221662474
@@ -122,6 +133,8 @@ zend_result dom_attr_value_write(dom_object *obj, zval *newval)
 {
 	DOM_PROP_NODE(xmlAttrPtr, attrp, obj);
 
+	dom_attr_value_will_change(obj, attrp);
+
 	/* Typed property, this is already a string */
 	ZEND_ASSERT(Z_TYPE_P(newval) == IS_STRING);
 	zend_string *str = Z_STR_P(newval);
diff --git a/ext/dom/document.c b/ext/dom/document.c
index 54a9e99a304..79149bef69b 100644
--- a/ext/dom/document.c
+++ b/ext/dom/document.c
@@ -1028,34 +1028,47 @@ Since: DOM Level 2
 */
 PHP_METHOD(DOMDocument, getElementById)
 {
-	zval *id;
 	xmlDocPtr docp;
-	xmlAttrPtr  attrp;
 	size_t idname_len;
 	dom_object *intern;
 	char *idname;
 
-	id = ZEND_THIS;
-	if (zend_parse_parameters(ZEND_NUM_ARGS(), "s", &idname, &idname_len) == FAILURE) {
-		RETURN_THROWS();
-	}
+	ZEND_PARSE_PARAMETERS_START(1, 1)
+		Z_PARAM_STRING(idname, idname_len)
+	ZEND_PARSE_PARAMETERS_END();
 
-	DOM_GET_OBJ(docp, id, xmlDocPtr, intern);
+	DOM_GET_OBJ(docp, ZEND_THIS, xmlDocPtr, intern);
 
-	attrp = xmlGetID(docp, BAD_CAST idname);
-
-	/* From the moment an ID is created, libxml2's behaviour is to cache that element, even
-	 * if that element is not yet attached to the document. Similarly, only upon destruction of
-	 * the element the ID is actually removed by libxml2. Since libxml2 has such behaviour deeply
-	 * ingrained in the library, and uses the cache for various purposes, it seems like a bad
-	 * idea and lost cause to fight it. Instead, we'll simply walk the tree upwards to check
-	 * if the node is attached to the document. */
-	if (attrp && attrp->parent && php_dom_is_node_connected(attrp->parent)) {
-		DOM_RET_OBJ((xmlNodePtr) attrp->parent, intern);
+	/* If the document has not been manipulated yet, the ID cache will be in sync and we can trust its result.
+	 * This check mainly exists because a lot of times people just query a document without modifying it,
+	 * and we can allow quick access to IDs in that case. */
+	if (!dom_is_document_cache_modified_since_parsing(intern->document)) {
+		const xmlAttr *attrp = xmlGetID(docp, BAD_CAST idname);
+		if (attrp && attrp->parent) {
+			DOM_RET_OBJ(attrp->parent, intern);
+		}
 	} else {
-		RETVAL_NULL();
-	}
+		/* From the moment an ID is created, libxml2's behaviour is to cache that element, even
+		 * if that element is not yet attached to the document. Similarly, only upon destruction of
+		 * the element the ID is actually removed by libxml2. Since libxml2 has such behaviour deeply
+		 * ingrained in the library, and uses the cache for various purposes, it seems like a bad
+		 * idea and lost cause to fight it. */
 
+		const xmlNode *base = (const xmlNode *) docp;
+		const xmlNode *node = base->children;
+		while (node != NULL) {
+			if (node->type == XML_ELEMENT_NODE) {
+				for (const xmlAttr *attr = node->properties; attr != NULL; attr = attr->next) {
+					if (attr->atype == XML_ATTRIBUTE_ID && dom_compare_value(attr, BAD_CAST idname)) {
+						DOM_RET_OBJ((xmlNodePtr) node, intern);
+						return;
+					}
+				}
+			}
+
+			node = php_dom_next_in_tree_order(node, base);
+		}
+	}
 }
 /* }}} end dom_document_get_element_by_id */
 
diff --git a/ext/dom/element.c b/ext/dom/element.c
index 73a7d142b75..c20bcb01308 100644
--- a/ext/dom/element.c
+++ b/ext/dom/element.c
@@ -183,7 +183,7 @@ zend_result dom_element_id_read(dom_object *obj, zval *retval)
 	return dom_element_reflected_attribute_read(obj, retval, "id");
 }
 
-static void php_set_attribute_id(xmlAttrPtr attrp, bool is_id);
+static void php_set_attribute_id(xmlAttrPtr attrp, bool is_id, php_libxml_ref_obj *document);
 
 zend_result dom_element_id_write(dom_object *obj, zval *newval)
 {
@@ -191,7 +191,7 @@ zend_result dom_element_id_write(dom_object *obj, zval *newval)
 	if (!attr) {
 		return FAILURE;
 	}
-	php_set_attribute_id(attr, true);
+	php_set_attribute_id(attr, true, obj->document);
 	return SUCCESS;
 }
 /* }}} */
@@ -352,6 +352,16 @@ static xmlNodePtr dom_create_attribute(xmlNodePtr nodep, const char *name, const
 	}
 }
 
+static void dom_check_register_attribute_id(xmlAttrPtr attr, php_libxml_ref_obj *document)
+{
+	dom_mark_ids_modified(document);
+
+	if (attr->atype != XML_ATTRIBUTE_ID && attr->doc->type == XML_HTML_DOCUMENT_NODE && attr->ns == NULL && xmlStrEqual(attr->name, BAD_CAST "id")) {
+		/* To respect XML's ID behaviour, we only do this registration for HTML documents. */
+		attr->atype = XML_ATTRIBUTE_ID;
+	}
+}
+
 /* {{{ URL: http://www.w3.org/TR/2003/WD-DOM-Level-3-Core-20030226/DOM3-Core.html#core-ID-F68F082
 Modern spec URL: https://dom.spec.whatwg.org/#dom-element-setattribute
 Since:
@@ -360,7 +370,6 @@ PHP_METHOD(DOMElement, setAttribute)
 {
 	zval *id;
 	xmlNode *nodep;
-	xmlNodePtr attr = NULL;
 	int name_valid;
 	size_t name_len, value_len;
 	dom_object *intern;
@@ -394,23 +403,28 @@ PHP_METHOD(DOMElement, setAttribute)
 		}
 
 		/* Can't use xmlSetNsProp unconditionally here because that doesn't take into account the qualified name matching... */
-		attr = (xmlNodePtr) php_dom_get_attribute_node(nodep, BAD_CAST name, name_len);
+		xmlAttrPtr attr = php_dom_get_attribute_node(nodep, BAD_CAST name, name_len);
 		if (attr != NULL) {
-			dom_remove_all_children(attr);
+			dom_attr_value_will_change(intern, attr);
+			dom_remove_all_children((xmlNodePtr) attr);
 			xmlNodePtr node = xmlNewDocText(attr->doc, BAD_CAST value);
-			xmlAddChild(attr, node);
+			xmlAddChild((xmlNodePtr) attr, node);
 		} else {
-			attr = (xmlNodePtr) xmlSetNsProp(nodep, NULL, name_processed, BAD_CAST value);
+			attr = xmlSetNsProp(nodep, NULL, name_processed, BAD_CAST value);
+			if (EXPECTED(attr != NULL)) {
+				dom_check_register_attribute_id(attr, intern->document);
+			}
 		}
 
 		if (name_processed != BAD_CAST name) {
 			efree(name_processed);
 		}
 	} else {
-		attr = dom_get_attribute_or_nsdecl(intern, nodep, BAD_CAST name, name_len);
+		xmlNodePtr attr = dom_get_attribute_or_nsdecl(intern, nodep, BAD_CAST name, name_len);
 		if (attr != NULL) {
 			switch (attr->type) {
 				case XML_ATTRIBUTE_NODE:
+					dom_attr_value_will_change(intern, (xmlAttrPtr) attr);
 					node_list_unlink(attr->children);
 					break;
 				case XML_NAMESPACE_DECL:
@@ -693,7 +707,10 @@ static void dom_element_set_attribute_node_common(INTERNAL_FUNCTION_PARAMETERS,
 
 	xmlAddChild(nodep, (xmlNodePtr) attrp);
 	if (!modern) {
+		dom_mark_ids_modified(intern->document);
 		php_dom_reconcile_attribute_namespace_after_insertion(attrp);
+	} else {
+		dom_check_register_attribute_id(attrp, intern->document);
 	}
 
 	/* Returns old property if removed otherwise NULL */
@@ -870,6 +887,8 @@ static void dom_set_attribute_ns_legacy(dom_object *intern, xmlNodePtr elemp, ch
 	int errorcode = dom_check_qname(name, &localname, &prefix, uri_len, name_len);
 
 	if (errorcode == 0) {
+		dom_mark_ids_modified(intern->document);
+
 		if (uri_len > 0) {
 			nodep = (xmlNodePtr) xmlHasNsProp(elemp, BAD_CAST localname, BAD_CAST uri);
 			if (nodep != NULL && nodep->type != XML_ATTRIBUTE_DECL) {
@@ -958,8 +977,11 @@ static void dom_set_attribute_ns_modern(dom_object *intern, xmlNodePtr elemp, ze
 	if (errorcode == 0) {
 		php_dom_libxml_ns_mapper *ns_mapper = php_dom_get_ns_mapper(intern);
 		xmlNsPtr ns = php_dom_libxml_ns_mapper_get_ns_raw_prefix_string(ns_mapper, prefix, xmlStrlen(prefix), uri);
-		if (UNEXPECTED(xmlSetNsProp(elemp, ns, localname, BAD_CAST value) == NULL)) {
+		xmlAttrPtr attr = xmlSetNsProp(elemp, ns, localname, BAD_CAST value);
+		if (UNEXPECTED(attr == NULL)) {
 			php_dom_throw_error(INVALID_STATE_ERR, /* strict */ true);
+		} else {
+			dom_check_register_attribute_id(attr, intern->document);
 		}
 	} else {
 		php_dom_throw_error(errorcode, /* strict */ true);
@@ -1270,20 +1292,16 @@ PHP_METHOD(DOMElement, hasAttributeNS)
 }
 /* }}} end dom_element_has_attribute_ns */
 
-static void php_set_attribute_id(xmlAttrPtr attrp, bool is_id) /* {{{ */
+static void php_set_attribute_id(xmlAttrPtr attrp, bool is_id, php_libxml_ref_obj *document) /* {{{ */
 {
-	if (is_id == 1 && attrp->atype != XML_ATTRIBUTE_ID) {
-		xmlChar *id_val;
-
-		id_val = xmlNodeListGetString(attrp->doc, attrp->children, 1);
-		if (id_val != NULL) {
-			xmlAddID(NULL, attrp->doc, id_val, attrp);
-			xmlFree(id_val);
-		}
-	} else if (is_id == 0 && attrp->atype == XML_ATTRIBUTE_ID) {
+	if (is_id && attrp->atype != XML_ATTRIBUTE_ID) {
+		attrp->atype = XML_ATTRIBUTE_ID;
+	} else if (!is_id && attrp->atype == XML_ATTRIBUTE_ID) {
 		xmlRemoveID(attrp->doc, attrp);
 		attrp->atype = 0;
 	}
+
+	dom_mark_ids_modified(document);
 }
 /* }}} */
 
@@ -1311,7 +1329,7 @@ PHP_METHOD(DOMElement, setIdAttribute)
 	if (attrp == NULL || attrp->type == XML_ATTRIBUTE_DECL) {
 		php_dom_throw_error(NOT_FOUND_ERR, dom_get_strict_error(intern->document));
 	} else {
-		php_set_attribute_id(attrp, is_id);
+		php_set_attribute_id(attrp, is_id, intern->document);
 	}
 }
 /* }}} end dom_element_set_id_attribute */
@@ -1340,7 +1358,7 @@ PHP_METHOD(DOMElement, setIdAttributeNS)
 	if (attrp == NULL || attrp->type == XML_ATTRIBUTE_DECL) {
 		php_dom_throw_error(NOT_FOUND_ERR, dom_get_strict_error(intern->document));
 	} else {
-		php_set_attribute_id(attrp, is_id);
+		php_set_attribute_id(attrp, is_id, intern->document);
 	}
 }
 /* }}} end dom_element_set_id_attribute_ns */
@@ -1367,7 +1385,7 @@ static void dom_element_set_id_attribute_node(INTERNAL_FUNCTION_PARAMETERS, zend
 	if (attrp->parent != nodep) {
 		php_dom_throw_error(NOT_FOUND_ERR, dom_get_strict_error(intern->document));
 	} else {
-		php_set_attribute_id(attrp, is_id);
+		php_set_attribute_id(attrp, is_id, intern->document);
 	}
 }
 
diff --git a/ext/dom/internal_helpers.h b/ext/dom/internal_helpers.h
index 0f67550898a..a19367dd477 100644
--- a/ext/dom/internal_helpers.h
+++ b/ext/dom/internal_helpers.h
@@ -63,4 +63,31 @@ DOM_DEF_GET_CE_FUNC(domimplementation)
 
 #endif
 
+static zend_always_inline size_t dom_minimum_modification_nr_since_parsing(php_libxml_ref_obj *doc_ptr)
+{
+	/* For old-style DOM, we need a "new DOMDocument" + a load, so the minimum modification nr is 2.
+	 * For new-style DOM, we need only to call a named constructor, so the minimum modification nr is 1. */
+	return doc_ptr->class_type == PHP_LIBXML_CLASS_MODERN ? 1 : 2;
+}
+
+static zend_always_inline void dom_mark_document_cache_as_modified_since_parsing(php_libxml_ref_obj *doc_ptr)
+{
+	if (doc_ptr) {
+		doc_ptr->cache_tag.modification_nr = MAX(dom_minimum_modification_nr_since_parsing(doc_ptr) + 1,
+												 doc_ptr->cache_tag.modification_nr);
+	}
+}
+
+/* Marks the ID cache as potentially stale */
+static zend_always_inline void dom_mark_ids_modified(php_libxml_ref_obj *doc_ptr)
+{
+	/* Although this is currently a wrapper function, it's best to abstract the actual implementation away. */
+	dom_mark_document_cache_as_modified_since_parsing(doc_ptr);
+}
+
+static zend_always_inline bool dom_is_document_cache_modified_since_parsing(php_libxml_ref_obj *doc_ptr)
+{
+	return !doc_ptr || doc_ptr->cache_tag.modification_nr > dom_minimum_modification_nr_since_parsing(doc_ptr);
+}
+
 #endif
diff --git a/ext/dom/node.c b/ext/dom/node.c
index 14992da9526..5200a4d3cc4 100644
--- a/ext/dom/node.c
+++ b/ext/dom/node.c
@@ -185,6 +185,7 @@ zend_result dom_node_node_value_write(dom_object *obj, zval *newval)
 	/* Access to Element node is implemented as a convenience method */
 	switch (nodep->type) {
 		case XML_ATTRIBUTE_NODE:
+			dom_attr_value_will_change(obj, (xmlAttrPtr) nodep);
 			if (php_dom_follow_spec_intern(obj)) {
 				dom_remove_all_children(nodep);
 				xmlAddChild(nodep, xmlNewTextLen(BAD_CAST ZSTR_VAL(str), ZSTR_LEN(str)));
diff --git a/ext/dom/php_dom.h b/ext/dom/php_dom.h
index 27c603a3d9d..1978e94d1b3 100644
--- a/ext/dom/php_dom.h
+++ b/ext/dom/php_dom.h
@@ -174,6 +174,7 @@ xmlDocPtr php_dom_create_html_doc(void);
 xmlEntityPtr dom_entity_reference_fetch_and_sync_declaration(xmlNodePtr reference);
 void dom_set_xml_class(php_libxml_ref_obj *document);
 bool dom_compare_value(const xmlAttr *attr, const xmlChar *value);
+void dom_attr_value_will_change(dom_object *obj, xmlAttrPtr attrp);
 
 typedef enum {
 	DOM_LOAD_STRING = 0,
diff --git a/ext/dom/tests/bug79701/id_property.phpt b/ext/dom/tests/bug79701/id_property.phpt
new file mode 100644
index 00000000000..035a6c82f0f
--- /dev/null
+++ b/ext/dom/tests/bug79701/id_property.phpt
@@ -0,0 +1,53 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - id property variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\XMLDocument::createFromString(<<<XML
+<root>
+    <test1/>
+    <test2/>
+</root>
+XML);
+
+$test1 = $dom->documentElement->firstElementChild;
+$test2 = $test1->nextElementSibling;
+
+echo "--- After parsing ---\n";
+var_dump($dom->getElementById("x")?->nodeName);
+
+echo "--- After setting test2 ---\n";
+$test2->id = "x";
+var_dump($dom->getElementById("x")?->nodeName);
+echo "--- After setting test1 ---\n";
+$test1->id = "x";
+var_dump($dom->getElementById("x")?->nodeName);
+echo "--- After resetting test1 ---\n";
+$test1->id = "y";
+var_dump($dom->getElementById("x")?->nodeName);
+echo "--- After resetting test2 ---\n";
+$test2->id = "y";
+var_dump($dom->getElementById("x")?->nodeName);
+echo "--- After resetting test1 ---\n";
+$test1->id = "x";
+var_dump($dom->getElementById("x")?->nodeName);
+echo "--- After calling setIdAttribute with false on test1 ---\n";
+$test1->setIdAttribute("id", false);
+var_dump($dom->getElementById("x")?->nodeName);
+?>
+--EXPECT--
+--- After parsing ---
+NULL
+--- After setting test2 ---
+string(5) "test2"
+--- After setting test1 ---
+string(5) "test1"
+--- After resetting test1 ---
+string(5) "test2"
+--- After resetting test2 ---
+NULL
+--- After resetting test1 ---
+string(5) "test1"
+--- After calling setIdAttribute with false on test1 ---
+NULL
diff --git a/ext/dom/tests/bug79701/node.phpt b/ext/dom/tests/bug79701/node.phpt
new file mode 100644
index 00000000000..1c473f44e90
--- /dev/null
+++ b/ext/dom/tests/bug79701/node.phpt
@@ -0,0 +1,18 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - attribute node variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\HTMLDocument::createEmpty();
+$element = $dom->createElement('foo');
+$dom->append($element);
+$attr = $dom->createAttribute('id');
+$attr->value = 'test';
+var_dump($dom->getElementById('test')?->nodeName);
+$element->setAttributeNode($attr);
+var_dump($dom->getElementById('test')?->nodeName);
+?>
+--EXPECT--
+NULL
+string(3) "FOO"
diff --git a/ext/dom/tests/bug79701/prepend.phpt b/ext/dom/tests/bug79701/prepend.phpt
new file mode 100644
index 00000000000..ceca9bac3f6
--- /dev/null
+++ b/ext/dom/tests/bug79701/prepend.phpt
@@ -0,0 +1,29 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - prepending variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = new DOMDocument();
+$root = $dom->createElement('html');
+$dom->appendChild($root);
+
+$el1 = $dom->createElement('p1');
+$el1->setAttribute('id', 'foo');
+$el1->setIdAttribute('id', true);
+
+$root->appendChild($el1);
+
+$el2 = $dom->createElement('p2');
+$el2->setAttribute('id', 'foo');
+$el2->setIdAttribute('id', true);
+
+$root->appendChild($el2);
+unset($el1, $el2);
+
+$root->removeChild($dom->getElementById('foo'));
+
+var_dump($dom->getElementById('foo')?->nodeName);
+?>
+--EXPECT--
+string(2) "p2"
diff --git a/ext/dom/tests/bug79701/remove_attribute.phpt b/ext/dom/tests/bug79701/remove_attribute.phpt
new file mode 100644
index 00000000000..88fcc38c456
--- /dev/null
+++ b/ext/dom/tests/bug79701/remove_attribute.phpt
@@ -0,0 +1,21 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - remove attribute variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\XMLDocument::createFromString(<<<XML
+<root>
+    <test1 xml:id="x"/>
+    <test2 xml:id="x"/>
+</root>
+XML);
+
+var_dump($dom->getElementById('x')?->nodeName);
+$dom->getElementById('x')->removeAttribute('xml:id');
+var_dump($dom->getElementById('x')?->nodeName);
+?>
+--EXPECTF--
+Warning: Dom\XMLDocument::createFromString(): ID x already defined in Entity, line: 3 in %s on line %d
+string(5) "test1"
+NULL
diff --git a/ext/dom/tests/bug79701/set_attr_value.phpt b/ext/dom/tests/bug79701/set_attr_value.phpt
new file mode 100644
index 00000000000..096e8b878c5
--- /dev/null
+++ b/ext/dom/tests/bug79701/set_attr_value.phpt
@@ -0,0 +1,45 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - nodeValue / value variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+foreach (["value", "nodeValue"] as $property) {
+    echo "--- Testing property \$$property ---\n";
+    $dom = Dom\XMLDocument::createFromString(<<<XML
+    <root>
+        <test1 xml:id="x"/>
+        <test2 xml:id="y"/>
+    </root>
+    XML);
+
+    $test1 = $dom->getElementById('x');
+    $test2 = $dom->getElementById('y');
+
+    var_dump($test1?->nodeName);
+    var_dump($test2?->nodeName);
+
+    $test1->getAttributeNode('xml:id')->$property = "y";
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    $test2->getAttributeNode('xml:id')->$property = "x";
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+}
+?>
+--EXPECT--
+--- Testing property $value ---
+string(5) "test1"
+string(5) "test2"
+NULL
+string(5) "test1"
+string(5) "test2"
+string(5) "test1"
+--- Testing property $nodeValue ---
+string(5) "test1"
+string(5) "test2"
+NULL
+string(5) "test1"
+string(5) "test2"
+string(5) "test1"
diff --git a/ext/dom/tests/bug79701/set_attribute_html.phpt b/ext/dom/tests/bug79701/set_attribute_html.phpt
new file mode 100644
index 00000000000..ff6a9d5019c
--- /dev/null
+++ b/ext/dom/tests/bug79701/set_attribute_html.phpt
@@ -0,0 +1,82 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - set attribute in html variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+foreach (["xml:id", "ID"] as $name) {
+    $dom = Dom\HTMLDocument::createFromString(<<<HTML
+    <p>
+        <em id="x">1</em>
+        <strong id="y">2</strong>
+    </p>
+    HTML, LIBXML_NOERROR);
+
+    echo "\n=== Attribute name $name ===\n\n";
+
+    $test1 = $dom->getElementById('x');
+    $test2 = $dom->getElementById('y');
+
+    echo "--- After resetting em's id ---\n";
+
+    $test1->setAttribute($name, 'y');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting strong's id ---\n";
+
+    $test2->setAttribute($name, 'x');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting em's id ---\n";
+
+    $test1->setAttribute($name, 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting strong's id ---\n";
+
+    $test2->setAttribute($name, 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- Get id z ---\n";
+
+    var_dump($dom->getElementById('z')?->nodeName);
+}
+?>
+--EXPECT--
+=== Attribute name xml:id ===
+
+--- After resetting em's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting strong's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting em's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting strong's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- Get id z ---
+NULL
+
+=== Attribute name ID ===
+
+--- After resetting em's id ---
+NULL
+string(2) "EM"
+--- After resetting strong's id ---
+string(6) "STRONG"
+string(2) "EM"
+--- After resetting em's id ---
+string(6) "STRONG"
+NULL
+--- After resetting strong's id ---
+NULL
+NULL
+--- Get id z ---
+string(2) "EM"
diff --git a/ext/dom/tests/bug79701/set_attribute_ns_html.phpt b/ext/dom/tests/bug79701/set_attribute_ns_html.phpt
new file mode 100644
index 00000000000..473b1d2aad5
--- /dev/null
+++ b/ext/dom/tests/bug79701/set_attribute_ns_html.phpt
@@ -0,0 +1,85 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - set attribute ns in html variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+function test($namespace) {
+    $dom = Dom\HTMLDocument::createFromString(<<<HTML
+    <p>
+        <em id="x">1</em>
+        <strong id="y">2</strong>
+    </p>
+    HTML, LIBXML_NOERROR);
+
+    $test1 = $dom->getElementById('x');
+    $test2 = $dom->getElementById('y');
+
+    echo "--- After resetting em's id ---\n";
+
+    $test1->setAttributeNS($namespace, "id", 'y');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting strong's id ---\n";
+
+    $test2->setAttributeNS($namespace, "id", 'x');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting em's id ---\n";
+
+    $test1->setAttributeNS($namespace, "id", 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting strong's id ---\n";
+
+    $test2->setAttributeNS($namespace, "id", 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- Get id z ---\n";
+
+    var_dump($dom->getElementById('z')?->nodeName);
+}
+
+echo "=== Test empty namespace ===\n\n";
+test("");
+echo "\n=== Test \"urn:x\" namespace ===\n\n";
+test("urn:x");
+?>
+--EXPECT--
+=== Test empty namespace ===
+
+--- After resetting em's id ---
+NULL
+string(2) "EM"
+--- After resetting strong's id ---
+string(6) "STRONG"
+string(2) "EM"
+--- After resetting em's id ---
+string(6) "STRONG"
+NULL
+--- After resetting strong's id ---
+NULL
+NULL
+--- Get id z ---
+string(2) "EM"
+
+=== Test "urn:x" namespace ===
+
+--- After resetting em's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting strong's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting em's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- After resetting strong's id ---
+string(2) "EM"
+string(6) "STRONG"
+--- Get id z ---
+NULL
diff --git a/ext/dom/tests/bug79701/set_attribute_xml.phpt b/ext/dom/tests/bug79701/set_attribute_xml.phpt
new file mode 100644
index 00000000000..437b00a6da0
--- /dev/null
+++ b/ext/dom/tests/bug79701/set_attribute_xml.phpt
@@ -0,0 +1,143 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - set attribute in xml variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+function test($dom, $fn) {
+    $test1 = $dom->getElementById('x');
+    $test2 = $dom->getElementById('y');
+
+    echo "--- After resetting test1's id ---\n";
+
+    $fn($test1, 'xml:id', 'y');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting test2's id ---\n";
+
+    $fn($test2, 'xml:id', 'x');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting test1's id ---\n";
+
+    $fn($test1, 'xml:id', 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- After resetting test2's id ---\n";
+
+    $fn($test2, 'xml:id', 'z');
+    var_dump($dom->getElementById('x')?->nodeName);
+    var_dump($dom->getElementById('y')?->nodeName);
+
+    echo "--- Get id z ---\n";
+
+    var_dump($dom->getElementById('z')?->nodeName);
+}
+
+function getNamespace($name) {
+    if (str_contains($name, 'xml:')) {
+        return 'http://www.w3.org/XML/1998/namespace';
+    }
+    return '';
+}
+
+$common_xml = <<<XML
+<root>
+  <test1 xml:id="x"/>
+  <test2 xml:id="y"/>
+</root>
+XML;
+
+echo "\n=== DOMDocument: setAttribute ===\n\n";
+
+$dom = new DOMDocument;
+$dom->loadXML($common_xml);
+test($dom, fn ($element, $name, $value) => $element->setAttribute($name, $value));
+
+echo "\n=== DOMDocument: setAttributeNS ===\n\n";
+
+$dom = new DOMDocument;
+$dom->loadXML($common_xml);
+test($dom, fn ($element, $name, $value) => $element->setAttributeNS(getNamespace($name), $name, $value));
+
+echo "\n=== Dom\\XMLDocument: setAttribute ===\n\n";
+
+$dom = Dom\XMLDocument::createFromString($common_xml);
+test($dom, fn ($element, $name, $value) => $element->setAttribute($name, $value));
+
+echo "\n=== Dom\\XMLDocument: setAttributeNS ===\n\n";
+
+$dom = Dom\XMLDocument::createFromString($common_xml);
+test($dom, fn ($element, $name, $value) => $element->setAttributeNS(getNamespace($name), $name, $value));
+?>
+--EXPECT--
+=== DOMDocument: setAttribute ===
+
+--- After resetting test1's id ---
+NULL
+string(5) "test1"
+--- After resetting test2's id ---
+string(5) "test2"
+string(5) "test1"
+--- After resetting test1's id ---
+string(5) "test2"
+NULL
+--- After resetting test2's id ---
+NULL
+NULL
+--- Get id z ---
+string(5) "test1"
+
+=== DOMDocument: setAttributeNS ===
+
+--- After resetting test1's id ---
+NULL
+string(5) "test1"
+--- After resetting test2's id ---
+string(5) "test2"
+string(5) "test1"
+--- After resetting test1's id ---
+string(5) "test2"
+NULL
+--- After resetting test2's id ---
+NULL
+NULL
+--- Get id z ---
+string(5) "test1"
+
+=== Dom\XMLDocument: setAttribute ===
+
+--- After resetting test1's id ---
+NULL
+string(5) "test1"
+--- After resetting test2's id ---
+string(5) "test2"
+string(5) "test1"
+--- After resetting test1's id ---
+string(5) "test2"
+NULL
+--- After resetting test2's id ---
+NULL
+NULL
+--- Get id z ---
+string(5) "test1"
+
+=== Dom\XMLDocument: setAttributeNS ===
+
+--- After resetting test1's id ---
+NULL
+string(5) "test1"
+--- After resetting test2's id ---
+string(5) "test2"
+string(5) "test1"
+--- After resetting test1's id ---
+string(5) "test2"
+NULL
+--- After resetting test2's id ---
+NULL
+NULL
+--- Get id z ---
+string(5) "test1"
diff --git a/ext/dom/tests/bug79701/swap.phpt b/ext/dom/tests/bug79701/swap.phpt
new file mode 100644
index 00000000000..261073f9160
--- /dev/null
+++ b/ext/dom/tests/bug79701/swap.phpt
@@ -0,0 +1,47 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - swapping variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = new DOMDocument;
+$dom->loadXML(<<<XML
+<root>
+    <test1 xml:id="x"/>
+    <test2 attr="x"/>
+</root>
+XML);
+
+$test1 = $dom->getElementById('x');
+
+echo "--- After parsing ---\n";
+var_dump($dom->getElementById('x')?->nodeName);
+echo "--- After adding a new id attribute ---\n";
+$dom->getElementsByTagName('test2')[0]->setIdAttribute('attr', true);
+var_dump($dom->getElementById('x')?->nodeName);
+echo "--- After removing the first id ---\n";
+$dom->getElementById('x')->remove();
+var_dump($dom->getElementById('x')?->nodeName);
+echo "--- After removing the second id ---\n";
+$dom->getElementById('x')->remove();
+var_dump($dom->getElementById('x')?->nodeName);
+echo "--- After re-adding the first id ---\n";
+$dom->documentElement->appendChild($test1);
+var_dump($dom->getElementById('x')?->nodeName);
+echo "--- After changing the first id ---\n";
+$test1->setAttribute('xml:id', 'y');
+var_dump($dom->getElementById('x')?->nodeName);
+?>
+--EXPECT--
+--- After parsing ---
+string(5) "test1"
+--- After adding a new id attribute ---
+string(5) "test1"
+--- After removing the first id ---
+string(5) "test2"
+--- After removing the second id ---
+NULL
+--- After re-adding the first id ---
+string(5) "test1"
+--- After changing the first id ---
+NULL
diff --git a/ext/dom/tests/bug79701/toggle.phpt b/ext/dom/tests/bug79701/toggle.phpt
new file mode 100644
index 00000000000..9b549c1534a
--- /dev/null
+++ b/ext/dom/tests/bug79701/toggle.phpt
@@ -0,0 +1,17 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - toggle variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\HTMLDocument::createFromString('<p id="test">foo</p>', LIBXML_NOERROR | LIBXML_HTML_NOIMPLIED);
+var_dump($dom->getElementById('test')?->nodeName);
+$dom->documentElement->toggleAttribute('id');
+var_dump($dom->getElementById('test')?->nodeName);
+$dom->documentElement->toggleAttribute('id');
+var_dump($dom->getElementById('test')?->nodeName);
+?>
+--EXPECT--
+string(1) "P"
+NULL
+NULL
diff --git a/ext/dom/tests/bug79701/unconnected.phpt b/ext/dom/tests/bug79701/unconnected.phpt
new file mode 100644
index 00000000000..fe22f499fb7
--- /dev/null
+++ b/ext/dom/tests/bug79701/unconnected.phpt
@@ -0,0 +1,16 @@
+--TEST--
+Bug #79701 (getElementById does not correctly work with duplicate definitions) - unconnected variation
+--EXTENSIONS--
+dom
+--FILE--
+<?php
+$dom = Dom\HTMLDocument::createEmpty();
+$element = $dom->createElement('foo');
+$element->setAttribute('id', 'test');
+var_dump($dom->getElementById('test')?->nodeName);
+$dom->append($element);
+var_dump($dom->getElementById('test')?->nodeName);
+?>
+--EXPECT--
+NULL
+string(3) "FOO"