* When the problem happens: give few sample threads. Screenshot will be good.
I hardly vote in any threads.. Goundamani thread, Udhaya Nidhi Stalin movie thread, TN Assembly Election, ICC World Cup - I surely have not voted in these threads. But I get the message that I have already voted.

* What's your browser, OS (win xp/windows 7/linux)
XP

* Does it happen on all polls (old & new)? Or happens randomly?
I have not checked that but all the above polls I have mentioned are from the old hub I think. Will check on the new polls too and let you know.

* Have you tried from any pc/workstation, and faced the same problem?
Nope I have not.

And I guess Senthil is right. The polls created before migration have closed and hence we get the message.