How to allow duplicate S/N when Subcontracting a service

Hi, I hope you are well.

We are currently attempting to send our product (unpainted car bumper) to a sub-contractor for a painting service this will then return another product as in our BOM known (painted car bumper).

The issue that we are having is that the painted car bumper is not allowed to have the same S/N as the unpainted car bumper. Although it is the same product and painting does not warrant the need for a new S/N

Please let me know if anyone can solve this problem,
Kind regards