I like the idea of the paper ballets that get scanned. That way there is always back-up to show how each precinct voted. As well ... I'm not a big fan of the mail in ballots. Seems to easy to falsify or even lose in the mail to me.
States that use vote-by-mail do not appear to have this problem.
In CA, the way vote-by-mail works is that when you register to vote, your signature is recorded. Then when you're mail ballot is received, it is first compared electronically to your signature on file. If that automated comparison is failed, it is then checked and compared by a human.
Under this system, you don't have to worry about someone voting multiple times because only one ballot is mailed per voter.