diff --git a/src/Extension/ExtensionRegistry.php b/src/Extension/ExtensionRegistry.php
index a316fe24..7d359b7b 100644
--- a/src/Extension/ExtensionRegistry.php
+++ b/src/Extension/ExtensionRegistry.php
@@ -60,7 +60,7 @@ class ExtensionRegistry
 
     private function getExtensionClasses(): array
     {
-        return $this->extensionClasses;
+        return array_unique($this->extensionClasses);
     }
 
     /** @return ExtensionInterface[] */