title: how to validate the facebook ID of a user to prevent from re-submitting again
body: <p>I have a picture contest using php which allows you to enter to upload or enter to vote. It also only allow one facebook user to upload one picture to the event at most.</p>
  
  <p>How can I validate their facebook ID with MySQL data, such that the same user who already submitted once (according to the fbID) is not able to click the enter contest button again and when clicked, a msg box will pop out saying "You have already submitted once"?</p>
  
  <p>Hope you guys will give suggestions. Thanks and appreciated all of your time for helping me.</p>