title: Prevent double request handling
body: <p>I'm working with a 3rd-party payment service that sends my user back to my site after doing a payment on their site. It sends the user back with javascript (<code>setTimeout</code> and <code>location='my_site.com?data'</code>), but there's also a button the user can click if it takes too long. The double request happens when the browser is already loading my site because of the javascript, and the user clicks the button.<br>
  Can I prevent the request is handled twice? Or should I simply handle it twice?  </p>
  
  <p>Right now, I show a "request is already handled" when the same request is handled twice, but because the two requests happen near simultaneously, the user never sees the page that says the request was correctly handled.</p>