diff --git a/apps/testing/appinfo/routes.php b/apps/testing/appinfo/routes.php
index 0ab6b42d55..a6324a2d1f 100644
--- a/apps/testing/appinfo/routes.php
+++ b/apps/testing/appinfo/routes.php
@@ -48,6 +48,11 @@ return [
 			'url' => '/api/v1/app/{appid}/{configkey}',
 			'verb' => 'DELETE',
 		],
+		[
+			'name' => 'FlushUsedUserNames#executeFlush',
+			'url' => '/api/v1/flushDupeUsernames',
+			'verb' => 'POST',
+		],
 		[
 			'name' => 'Locking#isLockingEnabled',
 			'url' => '/api/v1/lockprovisioning',
diff --git a/apps/testing/composer/composer/autoload_classmap.php b/apps/testing/composer/composer/autoload_classmap.php
index 96bc2b7b68..507e574531 100644
--- a/apps/testing/composer/composer/autoload_classmap.php
+++ b/apps/testing/composer/composer/autoload_classmap.php
@@ -10,6 +10,7 @@ return array(
     'OCA\\Testing\\AlternativeHomeUserBackend' => $baseDir . '/../lib/AlternativeHomeUserBackend.php',
     'OCA\\Testing\\AppInfo\\Application' => $baseDir . '/../lib/AppInfo/Application.php',
     'OCA\\Testing\\Controller\\ConfigController' => $baseDir . '/../lib/Controller/ConfigController.php',
+    'OCA\\Testing\\Controller\\FlushUsedUserNamesController' => $baseDir . '/../lib/Controller/FlushUsedUserNamesController.php',
     'OCA\\Testing\\Controller\\LockingController' => $baseDir . '/../lib/Controller/LockingController.php',
     'OCA\\Testing\\Controller\\RateLimitTestController' => $baseDir . '/../lib/Controller/RateLimitTestController.php',
     'OCA\\Testing\\Locking\\FakeDBLockingProvider' => $baseDir . '/../lib/Locking/FakeDBLockingProvider.php',
diff --git a/apps/testing/composer/composer/autoload_static.php b/apps/testing/composer/composer/autoload_static.php
index 15b4617183..f17ea253a6 100644
--- a/apps/testing/composer/composer/autoload_static.php
+++ b/apps/testing/composer/composer/autoload_static.php
@@ -25,6 +25,7 @@ class ComposerStaticInitTesting
         'OCA\\Testing\\AlternativeHomeUserBackend' => __DIR__ . '/..' . '/../lib/AlternativeHomeUserBackend.php',
         'OCA\\Testing\\AppInfo\\Application' => __DIR__ . '/..' . '/../lib/AppInfo/Application.php',
         'OCA\\Testing\\Controller\\ConfigController' => __DIR__ . '/..' . '/../lib/Controller/ConfigController.php',
+        'OCA\\Testing\\Controller\\FlushUsedUserNamesController' => __DIR__ . '/..' . '/../lib/Controller/FlushUsedUserNamesController.php',
         'OCA\\Testing\\Controller\\LockingController' => __DIR__ . '/..' . '/../lib/Controller/LockingController.php',
         'OCA\\Testing\\Controller\\RateLimitTestController' => __DIR__ . '/..' . '/../lib/Controller/RateLimitTestController.php',
         'OCA\\Testing\\Locking\\FakeDBLockingProvider' => __DIR__ . '/..' . '/../lib/Locking/FakeDBLockingProvider.php',
diff --git a/apps/testing/lib/Controller/FlushUsedUserNamesController.php b/apps/testing/lib/Controller/FlushUsedUserNamesController.php
new file mode 100644
index 0000000000..3faa9d300d
--- /dev/null
+++ b/apps/testing/lib/Controller/FlushUsedUserNamesController.php
@@ -0,0 +1,55 @@
+<?php
+
+declare(strict_types=1);
+
+/**
+ * @copyright Copyright (c) 2021 Lukas Reschke <lukas@statuscode.ch>
+ *
+ * @author Lukas Reschke <lukas@statuscode.ch>
+ *
+ * @license GNU AGPL version 3 or any later version
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ *
+ */
+
+namespace OCA\Testing\Controller;
+
+use OC\User\UsernameDuplicationPreventionManager;
+use OCP\AppFramework\Http\DataResponse;
+use OCP\AppFramework\OCSController;
+use OCP\IRequest;
+
+class FlushUsedUserNamesController extends OCSController {
+
+	/** @var UsernameDuplicationPreventionManager */
+	private $usernameDuplicationPreventionManager;
+
+	/**
+	 * @param string $appName
+	 * @param IRequest $request
+	 * @param UsernameDuplicationPreventionManager $usernameDuplicationPreventionManager
+	 */
+	public function __construct($appName,
+								IRequest $request,
+								UsernameDuplicationPreventionManager $usernameDuplicationPreventionManager) {
+		parent::__construct($appName, $request);
+		$this->usernameDuplicationPreventionManager = $usernameDuplicationPreventionManager;
+	}
+
+	public function executeFlush() : DataResponse {
+		$this->usernameDuplicationPreventionManager->cleanUp();
+		return new DataResponse();
+	}
+}
diff --git a/build/integration/features/bootstrap/Provisioning.php b/build/integration/features/bootstrap/Provisioning.php
index d613986df6..de5a211bfd 100644
--- a/build/integration/features/bootstrap/Provisioning.php
+++ b/build/integration/features/bootstrap/Provisioning.php
@@ -414,6 +414,15 @@ trait Provisioning {
 		$this->response = $client->put($fullUrl, $options);
 	}
 
+	private function flushDeletedUserList() {
+		$previousUser = $this->currentUser;
+		$this->currentUser = 'admin';
+		$this->sendingTo('POST', "/cloud/apps/testing");
+		$this->sendingTo('POST', "/apps/testing/api/v1/flushDupeUsernames");
+		$this->sendingTo('DELETE', "/cloud/apps/testing");
+		$this->currentUser = $previousUser;
+	}
+
 	/**
 	 * @When /^Deleting the user "([^"]*)"$/
 	 * @param string $user
@@ -908,6 +917,8 @@ trait Provisioning {
 	 * @AfterScenario
 	 */
 	public function cleanupUsers() {
+		$this->flushDeletedUserList();
+
 		$previousServer = $this->currentServer;
 		$this->usingServer('LOCAL');
 		foreach ($this->createdUsers as $user) {