[coagmano]

I've only worked on one US election (Colorado 2014), but they did check people off a database when they voted. So some places already do this. Unfortunately that database went down multiple times during the day, leading to long queues and delays for voters.

In Australia, we do a similar check, but using paper rolls and cross-check for duplicates afterwards. No computer to go down means no delays. If they find a duplicate they investigate, and if there's ever enough to make a difference, they would re-run the election in that area. Once we lost a box of votes and they had to re-run the election across a whole state to make sure.