title: Google Tag Manager causing page form to submit twice
body: <p>I have a webpage - which i guess is fairly standard.</p>
  
  <p>Basically something like:</p>
  
  <pre><code>&lt;form action="/mywebsite/mypage" method="post" novalidate="novalidate"&gt;
    &lt;div class="blah"&gt;                    
      &lt;input class="form-control" id="name" maxlength="80" name="name" tabindex="1" type="text" value=""&gt;
      &lt;!-- tonnes more inputs and labels and stuff --&gt;
      &lt;input type="submit" class="btn" value="Submit" tabindex="2"&gt;   
    &lt;/div&gt;
  &lt;/form&gt;
  </code></pre>
  
  <p>But when the submit button is pressed, the form is submitted twice (depending on browser).</p>
  
  <p>Firefox - works fine - submits once consistantly<br>
  Chrome - intermittent, mostly works, sometimes submits twice.<br>
  IE (Edge) - submits twice 100% of the time </p>
  
  <p>Managed to narrow it down to google tag manager.   When this is removed it works fine.</p>
  
  <p>So I have this script in my _layout page (master page, template page, whatever you call it)</p>
  
  <pre><code>&lt;!-- Google Tag Manager --&gt;
  &lt;noscript&gt;
      &lt;iframe src="//www.googletagmanager.com/ns.html?id=GTM-ABCDEF1"
              height="0" width="0" style="display: none; visibility: hidden"&gt;
      &lt;/iframe&gt;
  &lt;/noscript&gt;
  &lt;script&gt;
      (function(w, d, s, l, i) {
          w[l] = w[l] || [];
          w[l].push({
              'gtm.start':
                  new Date().getTime(),
              event: 'gtm.js'
          });
          var f = d.getElementsByTagName(s)[0],
              j = d.createElement(s),
              dl = l != 'dataLayer' ? '&amp;l=' + l : '';
          j.async = true;
          j.src =
              '//www.googletagmanager.com/gtm.js?id=' + i + dl;
          f.parentNode.insertBefore(j, f);
      })(window, document, 'script', 'dataLayer', 'GTM-ABCDEF1');
  &lt;/script&gt;
  
  &lt;!-- End Google Tag Manager --&gt;
  </code></pre>
  
  <p>So I'm trying to understand what's going on - why it would be causing it - and how to stop it.<br>
  I can work around it - but I don't want to. Would rather fix it properly.</p>
  
  <p>Notes:  yes for some reason the url starts // rather than https:// - but changing it makes no difference.  </p>
  
  <p>Any ideas?</p>
  
  <p>I have a very basic understanding of google analytics - but 0 knowledge about google tag manager.</p>