diff --git a/src/Template/Twig.php b/src/Template/Twig.php
index 1af591f..ec6461d 100644
--- a/src/Template/Twig.php
+++ b/src/Template/Twig.php
@@ -50,7 +50,17 @@ public function render(): string
      */
     public static function addExtension(\Twig\Extension\ExtensionInterface $extension): void
     {
-        self::twig()->addExtension($extension);
+        $twig = self::twig();
+
+        if ($extension instanceof \Twig\Extension\AttributeExtension) {
+            $extension_class = $extension->getClass();
+        } else {
+            $extension_class = $extension::class;
+        }
+
+        if (!$twig->hasExtension($extension_class)) {
+            self::twig()->addExtension($extension);
+        }
     }
 
     /**
diff --git a/src/Tests/ApplicationHelper.php b/src/Tests/ApplicationHelper.php
index e7da58b..b5d762a 100644
--- a/src/Tests/ApplicationHelper.php
+++ b/src/Tests/ApplicationHelper.php
@@ -51,10 +51,6 @@ trait ApplicationHelper
     #[\PHPUnit\Framework\Attributes\BeforeClass]
     public static function loadApplication(): void
     {
-        // This avoids to register extensions multiple times (normally done in
-        // the Application class constructor) as it would fail.
-        \Minz\Template\Twig::reset();
-
         $app_name = \Minz\Configuration::$app_name;
         $application_class_name = "\\{$app_name}\\Application";
         try {