Great question — answering as the UphillTech team:
Using only IP addresses for user authentication/identification is extremely unreliable. There might be many hundreds or even thousands of users behind one IP (e.g a corporate network) and for most of those on home connections their IPs are likely to be dynamic and regularly changing. You have to use Cookies for more reliable tracking. You can specify just about any time-to-live for a cookie, so that when an anonymous user returns, you can identify him. Of course cookies can be deleted by users, so they could delete their cookies and vote again. However, is this likely to be a big problem? If someone really wants to game your poll, they could write a script. However, you could add a few...
Full answer by Richard H: https://stackoverflow.stackexchange.com/a/1893128 (CC BY-SA 4.0).
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