Serial numbers are not necessarily unique. I know of a fellow whose Winchester lever action was seized as stolen property only to find out (two years later) that it was some commemorative Winchester with the same S/N that was stolen. He eventually got his rifle back but it was pretty seriously trashed, rusty and banged up stock.
Manufacturers are under no obligation to use unique serial numbers and can reuse S/Ns
I remember reading of a man who had a legally owned and licensed mint condition Thompson sub machine gun.
One day the local police came and confiscated the firearm. After several years of law suits he got his mint condition Thompson back, so rusted up it was trashed.