My situation is that I use serial number to manage my product, just like there is 6 camera serial number from 001-006.

After I sell 4 product and delivered, customer find out one of the camera (S/N 003) is defective, and ask for a replacement (S/N 005).

Then I find out original OUT inventory order and click return to return one product, from customer location to our warehouse, and mark serial number as S/N003.

System shows "The combination of serial number and product must be unique !" when I try to validate and comfirm this order.

Is there anything wrong?  What's the correct process if product replacement or order cancelled?  My Odoo version is 12.

Thanks.