Git commits in Moodle 3.7.x by Justus Dieckmann

commit 8516febdd7c11fc8d399765e821d35db7eff349e
Author: Justus Dieckmann <justusdieckmann@wwu.de>
Date:   Fri, 21 Dec 2018 03:19:38 +0800
Tag:    v3.7.0-beta

MDL-58256 group: prevents users to be added to a group twice