I can see people being register in two States. Is there a process to deregister from a State if you move to another state.
The real fraud is if you vote in both states.
My town in Massachusetts sends an annual form listing all the residents on record as residing at each address, and an indication of which residents are registered to vote.
You have to make corrections/additions, sign the form and return it. Failure to do so results in removal from voter registration files.
It has been suggested that many of the folks who winter in Florida vote there and at their summer homes as well.