Git commits in Moodle 3.5.5 by Justus Dieckmann

commit 609203ada6533296872b58f38fca1aac3f6bf2df
Author: Justus Dieckmann <justusdieckmann@wwu.de>
Date:   Tue, 15 Jan 2019 03:16:55 +0800
Tag:    v3.5.5

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