I don't care how much maths and encryption you use, you can't get out of the fact that things can be anonymous (no one can know how you voted) or verifiable (people can prove that you only voted once) but not both. - Switzerland usually gets around this by knowing where everyone lives and mailing them a piece of paper 'something you have' - South Africa gets around this by putting ink on your fingernail I've read qui…
It's not that there are no checks. You have to give your name, and they know if you've voted more than once at that station that day. To vote more than once you'd have to pretend to be somebody else, in person, which means that if you're caught you will go to jail.
We could certainly do better, but thus far all efforts to defeat this non-problem are clearly targeted at making it harder for people to vote rather than any kind of election integrity.