there's a much simpler solution for in person voting, used in Italy for example: you have a numbered sticker on the ballot, when you get the ballot the sticker id is written down, when you get out of the voting booth it gets verified and detached.

The ballot you throw in the box does not have the sticker (vote is anonymous) and you cannot come in with a pre-marked ballot (and being out one) cause the number would not match.