GH-745 Fix group enrolment

Fix incorrect comparison (group name vs group ID). Also: do not try to
users that are already members.

classes/catquiz.php

@@ -1979,7 +1979,10 @@ public static function enrol_and_create_message_array(
                 $groupsofcourse = groups_get_all_groups($courseid);
                 foreach ($groupsofcourse as $existinggroup) {
                     foreach ($groupstoenrol[$catscaleid] as $newgroup) {
-                        if ($existinggroup->id == $newgroup) {
+                        if ($existinggroup->name == $newgroup) {
+                            if (groups_is_member($existinggroup->id, $userid)) {
+                                continue;
+                            }
                             $groupmember = groups_add_member($existinggroup->id, $userid);
                             if ($groupmember) {
                                 $data = [];