Author: Petr Škoda
MDL-32572 detect username conflicts in auth_db sync
http://git.moodle.org/gw?p=moodle.git;a=commit;h=bee022093570261f50a367e6544986a487e464c0
http://github.com/moodle/moodle/commit/bee022093570261f50a367e6544986a487e464c0
+ 2.4 en [auth_dbinsertuserduplicate,auth_db]