Google Tag Manager causing page form to submit twice

Question

I have a webpage - which i guess is fairly standard.

Basically something like:

<form action="/mywebsite/mypage" method="post" novalidate="novalidate">
  <div class="blah">                    
    <input class="form-control" id="name" maxlength="80" name="name" tabindex="1" type="text" value="">
    <!-- tonnes more inputs and labels and stuff -->
    <input type="submit" class="btn" value="Submit" tabindex="2">   
  </div>
</form>

But when the submit button is pressed, the form is submitted twice (depending on browser).

Firefox - works fine - submits once consistantly
Chrome - intermittent, mostly works, sometimes submits twice.
IE (Edge) - submits twice 100% of the time

Managed to narrow it down to google tag manager. When this is removed it works fine.

So I have this script in my _layout page (master page, template page, whatever you call it)

<!-- Google Tag Manager -->
<noscript>
    <iframe src="//www.googletagmanager.com/ns.html?id=GTM-ABCDEF1"
            height="0" width="0" style="display: none; visibility: hidden">
    </iframe>
</noscript>
<script>
    (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' ? '&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');
</script>

<!-- End Google Tag Manager -->

So I'm trying to understand what's going on - why it would be causing it - and how to stop it.
I can work around it - but I don't want to. Would rather fix it properly.

Notes: yes for some reason the url starts // rather than https:// - but changing it makes no difference.

Any ideas?

I have a very basic understanding of google analytics - but 0 knowledge about google tag manager.

Answer 1

A bug in GTM, see https://productforums.google.com/forum/?nomobile=true#!topic/tag-manager/QVb2sNyvp5k;context-place=forum/tag-manager for more information. The selected answer isn't really the correct answer.