diff --git a/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php b/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
index 3d0a07667494..a474613ef7bb 100644
--- a/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
+++ b/src/Symfony/Component/DependencyInjection/Loader/FileLoader.php
@@ -112,7 +112,7 @@ public function registerClasses(Definition $prototype, $namespace, $resource, $e
                     continue;
                 }
                 foreach (class_implements($class, false) as $interface) {
-                    $this->singlyImplemented[$interface] = isset($this->singlyImplemented[$interface]) ? false : $class;
+                    $this->singlyImplemented[$interface] = ($this->singlyImplemented[$interface] ?? $class) !== $class ? false : $class;
                 }
             }
         }
diff --git a/src/Symfony/Component/DependencyInjection/Tests/Loader/FileLoaderTest.php b/src/Symfony/Component/DependencyInjection/Tests/Loader/FileLoaderTest.php
index b9ea798d2c5c..970cf416b344 100644
--- a/src/Symfony/Component/DependencyInjection/Tests/Loader/FileLoaderTest.php
+++ b/src/Symfony/Component/DependencyInjection/Tests/Loader/FileLoaderTest.php
@@ -91,6 +91,8 @@ public function testRegisterClasses()
         $loader = new TestFileLoader($container, new FileLocator(self::$fixturesPath.'/Fixtures'));
 
         $loader->registerClasses(new Definition(), 'Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\Sub\\', 'Prototype/%sub_dir%/*');
+        $loader->registerClasses(new Definition(), 'Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\Sub\\', 'Prototype/%sub_dir%/*'); // loading twice should not be an issue
+        $loader->registerAliasesForSinglyImplementedInterfaces();
 
         $this->assertEquals(
             ['service_container', Bar::class],
@@ -119,6 +121,7 @@ public function testRegisterClassesWithExclude()
             // load everything, except OtherDir/AnotherSub & Foo.php
             'Prototype/{%other_dir%/AnotherSub,Foo.php}'
         );
+        $loader->registerAliasesForSinglyImplementedInterfaces();
 
         $this->assertTrue($container->has(Bar::class));
         $this->assertTrue($container->has(Baz::class));
@@ -148,6 +151,8 @@ public function testRegisterClassesWithExcludeAsArray()
                 'Prototype/OtherDir/AnotherSub/DeeperBaz.php',
             ]
         );
+        $loader->registerAliasesForSinglyImplementedInterfaces();
+
         $this->assertTrue($container->has(Foo::class));
         $this->assertTrue($container->has(Baz::class));
         $this->assertFalse($container->has(Bar::class));
@@ -162,6 +167,7 @@ public function testNestedRegisterClasses()
         $prototype = new Definition();
         $prototype->setPublic(true)->setPrivate(true);
         $loader->registerClasses($prototype, 'Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\\', 'Prototype/*');
+        $loader->registerAliasesForSinglyImplementedInterfaces();
 
         $this->assertTrue($container->has(Bar::class));
         $this->assertTrue($container->has(Baz::class));
@@ -193,6 +199,7 @@ public function testMissingParentClass()
             'Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\BadClasses\\',
             'Prototype/%bad_classes_dir%/*'
         );
+        $loader->registerAliasesForSinglyImplementedInterfaces();
 
         $this->assertTrue($container->has(MissingParent::class));
 
@@ -211,6 +218,7 @@ public function testRegisterClassesWithBadPrefix()
 
         // the Sub is missing from namespace prefix
         $loader->registerClasses(new Definition(), 'Symfony\Component\DependencyInjection\Tests\Fixtures\Prototype\\', 'Prototype/Sub/*');
+        $loader->registerAliasesForSinglyImplementedInterfaces();
     }
 
     public function testRegisterClassesWithIncompatibleExclude()
@@ -226,6 +234,7 @@ public function testRegisterClassesWithIncompatibleExclude()
             'Prototype/*',
             'yaml/*'
         );
+        $loader->registerAliasesForSinglyImplementedInterfaces();
     }
 }
 
@@ -240,10 +249,4 @@ public function supports($resource, $type = null): bool
     {
         return false;
     }
-
-    public function registerClasses(Definition $prototype, $namespace, $resource, $exclude = null)
-    {
-        parent::registerClasses($prototype, $namespace, $resource, $exclude);
-        $this->registerAliasesForSinglyImplementedInterfaces();
-    }
 }