I suggest that you put a condition on the Action so it does not try to duplicate a Key field. I'm not quite sure I understand your setup in terms of duplicating the Key field, but it sounds like you have a 1 for 1 relationship where every parent has exactly 1 child record and you have set the Key field of the Child to be the same as the Record ID# of the Parent.
If that is the case, then you should be able to do a summary field of the # of children, and not fire the Action of the Parent already has it's child record created.