Q:

Number of times you can vote for particular person

I have to imagine the creators of these sites built a mechanism to detect "Vote Fraud".  Is there something to stop someone from creating two accounts and continuously vote for each other to build reputation?

A:

Yeap. It is covered in this blog entry: "Vote Fraud and You" and this one: "More Voting anomalies" on blog.stackoverflow.com
Basically SO query the database from time to time and if they find that a user is just upvoting some other specific user, they all the points from that user are removed.

