I would bet that many of them never contacted the county election office when they moved from one state to the other.
When I moved from CA to VA and registered to vote, the VA elections office notified its CA counterpart that I was now registered in VA and that I should be taken off the CA rolls. That was before the internet when the notification involved paper and postage. I would imagine such notifications would be relatively easy in the electronic age.
Most states have a “missed vote” rule that removes people from roles automatically after theý’ve not voted a few, usually five, times. That’s the proof that these scum bagz are indeed multiple-voting. If they had moved from one state to the other and weren’t cheating, then their old registration would have expired.
One way to combat this would be to require people to state where/if they were previously registered when they attempt to register in a new town/state etc. With lying a felony.