In our case we are using addMember to add users to the group, when group membership request is approved, but it was discovered that we can add like users to multiple groups, even the cardinality set to "1".
This happens, because addRelationship doesn't validate the entity before saving it and thus we skip all validations for entity.
$storage = $this->relationshipStorage();
$relationship = $storage->createForEntityInGroup($entity, $this, $plugin_id, $values);
$storage->save($relationship);
return $relationship;
Validate an entity before saving it, I am unsure what the best approach would be for the Group module.
Active
3.3
Code
After looking into this issue I noticed that this is also true for the native joining process.
So if users are allowed to join a group they can join multipe groups of the same type even though the group cardinality for the group membership plugin is set to 1.