diff --git a/data/web/json_api.php b/data/web/json_api.php
index c3f8d9fc6..dfaa7383c 100644
--- a/data/web/json_api.php
+++ b/data/web/json_api.php
@@ -465,11 +465,18 @@ if (isset($_GET['query'])) {
         // webauthn two factor authentication
         case "webauthn-tfa-registration":
           if (isset($_SESSION["mailcow_cc_role"])) {
+              // Exclude existing CredentialIds, if any
+              $stmt = $pdo->prepare("SELECT `keyHandle` FROM `tfa` WHERE username = :username");
+              $stmt->execute(array(':username' => $_SESSION['mailcow_cc_username']));
+              $rows = $stmt->fetchAll(PDO::FETCH_ASSOC);
+              while($row = array_shift($rows)) {
+                $excludeCredentialIds[] = base64_decode($row['keyHandle']);
+              }
               // getCreateArgs($userId, $userName, $userDisplayName, $timeout=20, $requireResidentKey=false, $requireUserVerification=false, $crossPlatformAttachment=null, $excludeCredentialIds=array())
               // cross-platform: true, if type internal is not allowed
               //        false, if only internal is allowed
               //        null, if internal and cross-platform is allowed
-              $createArgs = $WebAuthn->getCreateArgs($_SESSION["mailcow_cc_username"], $_SESSION["mailcow_cc_username"], $_SESSION["mailcow_cc_username"], 30, false, $GLOBALS['WEBAUTHN_UV_FLAG_REGISTER'], null);
+              $createArgs = $WebAuthn->getCreateArgs($_SESSION["mailcow_cc_username"], $_SESSION["mailcow_cc_username"], $_SESSION["mailcow_cc_username"], 30, false, $GLOBALS['WEBAUTHN_UV_FLAG_REGISTER'], null, $excludeCredentialIds);
               
               print(json_encode($createArgs));
               $_SESSION['challenge'] = $WebAuthn->getChallenge();