I do not rely on cookies to ensure uniqueness. The cookie is used as a display hint to know when to show the results. Without the cookie you're able go through the motions, but your vote will not be tallied if you have previously registered a vote in a result set.
Two result sets are kept. One for each distinct screen name and another for each distinct IP address. As currently implemented, your vote is tallied in only one of the result sets, screen name if you're logged in, IP address if not. These two result sets correspond to "member" and "non-member" opinions in the software. I'm assuming if you're not logged in you're not "a member" (i.e. a registered user with a screen name.)
This means you can vote under your screen name, clear all your cookies, and vote again for your IP address. And you are able to cast votes for as many distinct IP addresses and distinct screen names you can acquire. But even though no error message is given, only one unique screen name or one unique IP address per poll is recorded.
John, I nominate you head of the Federal Elections Cookie Commission!
Thanks and kudos on seeking to ensure polling integrity!
Thanks, John. So you're polls are quite accurate -- unless a group from the DUmpster expends lots of time & energy to try to skew the results, which is not too likely to happen even for them.