How can I prevent a user to vote more than one time?

Programming contest and voting system is always problematic.

You have to make sure that a person can only vote 1 time.

How do you do that is the person is not login?

• IP address validation is not safe because a company can have the same external IP for all employe.
• IP + browser signature could be a better solution

// The connection() function returns the IP address of the user