Persistent cookies can easily be cleared. IP address limitations might prevent legitimate users behind firewalls, while allowing others to vote multiple times i

Persistent cookies can easily be cleared. IP address limitations might prevent legitimate users behind firewalls, while allowing others to vote multiple times if they have access to multiple computers. I've looked into the problem for my own voting site (https://modernballots.com/ https://modernballots.com/) and I haven't found a good way of doing this for public polls where anyone's allowed to vote. The solution I developed was to just create the option for private elections that require invitations to vote in. This has its own issues (most notably the distribution of invitation keys), but it's worked so far. Long story short, there's no good way of doing what you're looking for unless you're willing to accept the risk of false positives, false negatives, and the problems that come with both.