To me, that seems like a pretty effective low-tech way to prevent people from voting more than once...
I was thinking of a similar solution. When you go to clubs that charge admission, they ink stamp your hand. Why dont we stamp everyone as soon as they enter a polling place? If they try to vote again or go to other voting locations, just check their hand at the door to see if theyre already stamped. We could use security ink (invisible ink) with an inexpensive ultraviolet light which would make the stamp visible.