[BUGFIX] EXT:sys_action: prepend prefix only once on BE-User edit

Change-Id: I4e00d5a2198f6f3f98f8a08161188d5982962290
Resolves: #27440
Resolves: #31110
Related: #68370
Releases: master
Reviewed-on: http://review.typo3.org/41712


Reviewed-by: Wouter Wolters <typo3@wouterwolters.nl>
Tested-by: Wouter Wolters <typo3@wouterwolters.nl>
Reviewed-by: Christian Kuhn <lolli@schwarzbu.ch>
Tested-by: Christian Kuhn <lolli@schwarzbu.ch>

typo3/sysext/sys_action/Classes/ActionTask.php

@@ -490,7 +490,11 @@ class ActionTask implements \TYPO3\CMS\Taskcenter\TaskInterface {
 	 * @return string Combined username
 	 */
 	protected function fixUsername($username, $prefix) {
-		return trim($prefix) . trim($username);
+		$prefix = trim($prefix);
+		if (substr($username, 0, strlen($prefix)) === $prefix) {
+			$username = substr($username, strlen($prefix));
+		}
+		return $prefix . $username;
 	}
 
 	/**