Perhaps I don't understand your thought process, but how could anyone have a matching bill that way? If the DJ was reading off a serial number, the only person to have a match would be the DJ himself, since they are unique.

Afaik, the only time serial numbers get reused is when a bill is reissued because it is damaged or worn, in which case they would shred the old bill (also, the new bill would get a neat star next to its serial number).