seat dupes possible?

Seriously now, how are seat dupes possibly?

Rarely do we push back from the gate on a "very, very full flight" (as the FAs and GAs say) without a red coat having to board to resolve some seat dupes early on. It's not late enough and close to push back that it's standby seat assignments when the original owner showed up, and very rarely does someone have to deplane to resolve it.

With modern computing and network technology, how is it possible to ever seat dupe? I don't understand how the airline computer systems could ever allow a boarding pass to be printed for a seat that has already been assigned on that flight, route, and time.