Author: Frederic Massart
MDL-45579 group: Prevent duplicate enrolment keys in course groups
http://git.moodle.org/gw?p=moodle.git;a=commit;h=ac2015f2cef935aebbaec7e8586303ebb7ec1e49
http://github.com/moodle/moodle/commit/ac2015f2cef935aebbaec7e8586303ebb7ec1e49
+ 2.7 en [enrolmentkeyalreadyinuse,core_group]