I told you the main energizing mode. Why are you evading this?
Anyway, my suggestion is that we just take a thumbprint of every voter (right next to their signature) and then if you find some screwball who has invested the time to vote twice, you will have proof in the form of two identical thumbprints. And, then you can avoid all these crazy lawsuits about which identification papers should be acceptable.
Sometimes common sense leads us to the best answers.