It's no 100% solution, but you can use a browser fingerprint in combination with the IP adress. See this site for some usable and easily gettable browser properties.
Disadvantages: Some people may be left out (especially in large organizations with a very restrictive and thus homogeneous infrastructure), others may vote twice, for example by using different browsers.
If you want a 95% solution you have to require people to sign up with their email adress and proving that they received the email by klicking an embedded link, but depending on how much interest they take in voting, it may scare off a lot of potential voters.
A 100% solution for this problem does not exist, as far as I'm aware of.
Edit: Cookies are another obvious choice if you don't care too much about people gaming the poll system (just writing an auto-voter that ignores the cookies you send it).
It's no 100% solution, but you can use a browser fingerprint in combination with the IP adress. See this site for some usable and easily gettable browser properties.
Disadvantages: Some people may be left out (especially in large organizations with a very restrictive and thus homogeneous infrastructure), others may vote twice, for example by using different browsers.
If you want a 95% solution you have to require people to sign up with their email adress and proving that they received the email by klicking an embedded link, but depending on how much interest they take in voting, it may scare off a lot of potential voters.
A 100% solution for this problem does not exist, as far as I'm aware of.
Edit: Cookies are another obvious choice if you don't care too much about people gaming the poll system (just writing an auto-voter that ignores the cookies you send it).