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.

> 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.

They did that in my country at scale. Physical presence is a must-have if you ask me.

Some time ago every election or referendum simply put a stamp in the passport when voting, but that was before plastic ID cards. Now they have an online verification process before handing you the ballot papers; this also reports your ID for the invalidation of any pre-election votes (e.g. mail-in ballots) elsewhere.

You used to need a passport to vote? 42-47% of Americans have passports currently. In 1990, only 5% of Americans had passports.

That comes from a time/place where it was issued to 100% adult citizens and it was illegal not to have it - i.e. you have a duty to get one when you come of age or become a citizen or your old one expires, and it's a misdemeanor with some fines if you don't, plus quite a few other legal interactions will be simply refused until you get that sorted out.

Now an ID card is a valid option so some people have only the ID card and not a proper passport; but there is an expectation that everyone (who isn't e.g. actively hiding from the authorities as an outlaw) would have a valid ID and if they don't it's acceptable to require that they get one before they can fully interact with the rest of the society e.g. government, banking, driving, owning real estate or cars, any legal contracts such as rent or credit, etc.

> Now they have an online verification process before handing you the ballot papers

How can a voting observer ensure that double voting is actually blocked by this system? Government can whitelist people 10x-voting the right way (real case from Russia).

Issuing 10 passeports to these people is orders of magnitude harder to scale and will have negative consequences (fraud elsewhere) a government is typically less willing to take.