Author: Frederic Massart
MDL-45579 group: Prevent duplicate enrolment keys in course groups
http://git.moodle.org/gw?p=moodle.git;a=commit;h=624458a94003065af637c5739ad774f3f7da48e8
http://github.com/moodle/moodle/commit/624458a94003065af637c5739ad774f3f7da48e8
+ 2.8 en [enrolmentkeyalreadyinuse,core_group]