title: Can't get access token using Facebook OAuth
body: <p>About week ago Facebook login on my website worked correctly, but now when I'm trying retrieve access token using "code" I get error "<strong>This authorization code has been used</strong>". I've checked a lot of docs and suggestions, but without any result.</p>
  
  <p>Steps to reproduce(used FB PHP SDK):</p>
  
  <ol>
  <li><p>Send user to FB auth with correct urlencoded return uri(with trailing slash):</p>
  
  <pre><code>https://www.facebook.com/dialog/oauth?client_id=[my_cid]&amp;redirect_uri=http%3A%2F%2Fmydomain.com%2Fauth%2Ffacebook%2F&amp;state=e15e0894cd922098f9f8f340c820538d&amp;scope=email&amp;display=page
  </code></pre></li>
  <li><p>Receiving code param on my redirect uri(everything goes fine)</p></li>
  <li><p>Sending request for access token:</p>
  
  <pre><code>https://graph.facebook.com/oauth/access_token?client_id=[my_cid]&amp;redirect_uri=http%3A%2F%mydomain.com%2Fauth%2Ffacebook%2F&amp;client_secret=[my_secret]&amp;code=[long_code_goes_here]
  </code></pre></li>
  </ol>
  
  <p>I tried also not urlencoded redirect_uri.</p>
  
  <p>Why?</p>