I’m sure there are plenty of AKs with duplicated serial numbers all over the world, given all the different manufacturers. Most people aren’t going to look for details like markings from Zastava, Izhevsk, or Chinese factory #19.
FN ran into a similar problem with their FAL rifles, since they produced them for so many countries and the contract runs generally began with serial number 1...they began including a separate number on the rifle receiver that was the total production by FN. This was used to trace the origin of some of the FALs that appeared in the most recent Syrian civil war.
I want to trade two L1A1s for two actual FALs.