i'm not 100% sure, but i'm assuming a clone vote is when someone has or makes several user accounts to vote for themselves. There are a few ways of checking this out, IP address would most likely be primary, however, accounts who's only activity is voting on contests or accounts that were just made and then voted and thats all would be very good considerations as well. I can see how this would be a real pain if there were a lot of them, so perhaps there are :)