With GiftTool, you can ensure one vote per person. Although, the GiftTool system actually records all voting attempts - each attempt is kept separate.

Therefore to view your final election results, just sort your report by the number of attempts, restricting results to the first ballot submitted by each voter. In your instructions to your voters, you can let them know that only their first completed ballot would be counted (e.g. after they hit 'submit' for the first time) and any other subsequent ballots would be automatically discarded.