I've read Multiple accounts being used to dodge a question ban, but the multiple account user has same name.
What I'm thinking is that I cannot know if a user has multiple accounts with different names and up-vote each other. I think that, if s(he) has 5 or more accounts, she will be able to close a question if s(he) wants.
I found a comment likes "I thought that tracking of IP addresses would stop someone from doing this." How about if the multiple account users use difference locations, and difference IP addresses?