Create a duplicates query (use the wizard). In the subform BeforeInsert event - also can be done when they select the behaviour in both places (I would remove the top combobox, by the way, it serves no purpose at all, it does exactly the same as what the other one does, so why have it? Just adds unnecessary complexity). Do a dlookup on that query. If the behaviour is found then prevent them from adding it (in the BeforeInsert say Cancel=true)