Make invoice name uniqueness optional after a warning

We understand why you dont allow duplicate invoice numbers to be saved, however we have some suppliers who refuse to change from a long established practice of invoicing for multiple orders on a single invoice. This means we have to link the same invoice to multiple orders. At the minute we have to put in some (a)(b) to make them different but then they dont tie up automatically in our accounts software. Could we please override the safeguard when we need to?