The snowbirds voted in both states in 2000. The bigger numbers came from NJ and NY. There should be a nice big fine for doing it.
I suggest a fine of $25,000 and public listing of the perps. On an internet site. That way, Election officials in all states can reference the site for information.
Fine? How about some prison time (a felony conviction) with a fine to top it off? Just let them know that sure you can vote twice in one election, but it means you won't be voting in many other elections at all for a long time to come.