Author: Frederic Massart
MDL-45579 group: Prevent duplicate enrolment keys in course groups
http://git.moodle.org/gw?p=moodle.git;a=commit;h=e2f577e074879b163ee2dff919a8e2ad8b972560
http://github.com/moodle/moodle/commit/e2f577e074879b163ee2dff919a8e2ad8b972560
+ 2.6 en [enrolmentkeyalreadyinuse,core_group]