Author: Damyon Wiese
MDL-58774 auth_oauth2: Prevent duplicate linked logins
http://git.moodle.org/gw?p=moodle.git;a=commit;h=4f705f5d0d2ec7daf6b384e5d9281e2d37507cb5
http://github.com/moodle/moodle/commit/4f705f5d0d2ec7daf6b384e5d9281e2d37507cb5
+ 3.3 en [alreadylinked,auth_oauth2]