Ah, yes, this is indeed a tricky situation and, tbh, one I have not yet thought about. I don't think re-assigning domains to groups they have not been assigned before is a good idea but showing a warning can in general be done for certain partial imports where these situations may happen. It would at least ease maneuvering yourself back out of this situation easier. I don't think it can be avoided in a meaningful way with still offering the possibility to import only partially.