Users can be added to a group twice

Changes are made to the database during an upgrade. So begin your test in an unpatched version! Add "sleep(5);" into group/lib.php on line 100 to simulate a slow server. Navigate into a course with at least one enrolled user. Navigate to groups management and create a new, empty group. Select that group and click "Add/remove users" Open the same page you're at in a different browser. Select the same user on the right in both browsers. Quickly click "Add" in both browsers. Click "Back to groups". There should now be an incorrect member count, i.e. two members in the group "(2)" (remember: The patch must not have been applied before this step! Otherwise you will get a different result). Apply the patch and upgrade Moodle accordingly. Now there should be only one member in the group. "(1)". Repeat steps 2-8. With the patch, there should be only one member in the new group and one browser should have shown an error.