You should apply a large permanent mark to people that already voted. It must last one day or so. But that could infringe the right of not to vote, unless voting is mandatory. Or make impossible to vote twice because everybody is tied to exactly one anonymous ballot. See my reply to parent, about the voting system in Italy. However if some party control a part of the voting system, they can do whatever they want in several ways. For example vote with the ballots of people that didn't go to vote. In my country they'll have to get the photo id number of those people but it's not difficult to get if they have access to official data.