Building a contest app, any recommendations on how to track users?

[eluser]dallen33[/eluser]
I'm building a contest application that will mainly be used by mobile phones. Someone scans a code, and is brought to either a loser page or a winner page. We only want one entry per person for the duration of the contest. Of course, tracking the IPs is one way of doing it (storing IP in database). But what if the user's IP is dynamic? What could be another way of tracking a user? Do most mobile phones have cookies enabled?

[eluser]tomdelonge[/eluser]
There is no way to guarantee only one entry per person. IP address are most of the time dynamic (I think). Plus, they're mobile. Cookies can't be trusted, since someone could tamper with them (or in the least just remove them). The only good way of making sure someone doesn't do something twice is to require a sign in (which sounds too cumbersome for what you're doing) And even then, what's to stop multiple sign-ups? Another possibility is requiring the user's phone number.