What would be the best way to implement a "duplicate check" when users are recording a supplier invoice?

The aim would be to avoid accountants entering the same invoice twice, based on the supplier invoice number.

It shouldn't prevent them from entering the invoice - just warn them that an existing invoice is already in the system with the same number.

constraints prevent duplicate entry, the objective here is just to detect a potential duplicate and warn the user - letting them enter it again if they need to.