Let's say that I buy something at a store with an obvious counterfeit bill. If the cashier misses it, the person who counts the money at the end of the day will find it. If somehow they miss it, the bank they make the deposit at will find it.
It is the same with counterfeit shares!
They ultimately have to appear in someone's brokerage account...
If the share total in all brokerage accounts exceeds the OS, clearly there are counterfeit shares in circulation!