Developer credits
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