Well, there is no such security to preventing a double post. Even with unique Identification login can be duplicated by making two or more IDs. Not to mention ISP Ip address. Thus if they blocked the iSP Ip then there will only few people to vote for the poll. If they do it by UserName and IP then what about the different IP with different username? On the otherside it is impossible to have one vote per person because this is just how the nature the internet voting is.