There needs to be some way to stop snowbirds and students from being able to vote in two places.
Id say Voter ID is a good place to start. If every state sets up its own system, it should be possible to flag possible duplicate registrations through an interstate network, and the states involved can investigate further to determine if there is a duplicate registration, and if it should be purged.
Voter ID would also help identify same day registration fraud, when the person shows up at eight different precincts to vote, and each one of those registrations gets flagged. Since the state would have the voter ID data, finding the person afterward would be far easier than it is now (impossible)