title: What can cause a double page request?
body: <p>I am currently investigating a double request problem on my site. Not all the time, but sometimes, a requested page will in fact load twice...which is not a problem really until it is on a page with PHP that inserts stuff into my db on request (my tracking script).</p>
  
  <p>I have read that an empty src in an image tag, and an empty url() in a css background could potentially cause the page to be requested twice.</p>
  
  <p>However, I can't find any problems with those.</p>
  
  <p>Is there anything else that could be causing something like this?</p>
  
  <p><strong>ANSWER FOR MY SITUATION</strong></p>
  
  <p>After some extensive research, it turns out that in my case specifically, the second request has been coming from the user agent "Mediapartner-Google". I began to notice that on pages that serve an Adsense ad, I could expect a secondary visit from this crawler within seconds after I visit the page myself.</p>
  
  <p>This doesn't seem to be happening on pages without Adsense ads.</p>
  
  <p>I am going to mark an answer below, because it seems like for most situations, those are the correct things to check.</p>