You might need what I call a staging table - one that mimics the design of the final table (except for the ID field, such as the autonumber ID field). The staging table will allow a copy of the record, where you change whatever value in whichever field that will maintain the uniqueness of the combination of fields. You might need form level validation but that's not clear from your post. You could test for the uniqueness of the combination in the staging table compared to the final table after the staging table record is edited, or you could simply attempt to update/append the new combination to the final table and trap any resulting error if the attempt fails.
Or maybe you could figure out why whatever you're doing is attempting to save a record with the copied fields. If you copy a record that doesn't pass validation, you need to provide the opportunity to edit that record before you do anything that will attempt a save. Also, there is some ambiguity when you use the term combo in your post, as it could be taken to mean the combination of some fields, or it could refer to a combo box control.
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.