title: Prevent double voting
body: <p>I'm creating a web application where users will vote for some candidates by clicking thumbs up or thumbs down, and these users won't have any account on the site.</p>
  
  <p>What is the best technique to use? Is it necessary to use captcha for more protection from spam?</p>
  
  <p>Vote counts are expected to be millions, and the subject is not very critical as long as I get 95% accuracy that would be fine. Thanks.  </p>