Its quite simple really. People cannot vote more than once using the same account as the voting is not cookie based it is account based. We do not allow guests to vote as they dont have accounts. If someone has multiple accounts then yes, theoretically, they can vote more than once by logging in to the different accounts and voting.
As for detection - its manual, we simply check that the same voter has not selected the same option more than once. Its usually a mistake, but if we find any user abusing it or trying to rig the polls by registering multiple accounts we can and will remove the ability to vote in polls on a user by user basis. We pretty well know which users have multiple accounts and what usernames they go under so we will keep an eye out for this type of 'freeping'
Its not a perfect system, and for us, its not as easy as the system we used last year, but until something better comes along, it will do.