(function (i, s, o, g, r, a, m) {
    window['GoogleAnalyticsObject'] = 'ga';
    window['ga'] = window['ga'] || function () {
        (window['ga'].q = window['ga'].q || []).push(arguments)
    },
    window['ga'].l = 1 * new Date();
    a = document.createElement('script'),
    m = document.getElementsByTagName('script')[0];
    a.async = 1;
    a.src = '//www.google-analytics.com/analytics.js';
    m.parentNode.insertBefore(a, m)
})(window, document, 'script', '//www.google-analytics.com/analytics.js', 'ga');

ga('create', 'UA-xxxxxxxx-x', 'xxxxxx.com');
ga('send', 'pageview');
This is the Google Analytics (from now on GA) code snippet given to website admins/devs that enables GA for a given UA-number. Adding GA to our internal web app today, I noticed that the script Google provides doesn't actually have a mechanism to prevent people from adding duplicate scripts to their pages. I tested it and saw it was true - if the snippet was included twice, we had two scripts inserted to the DOM as the below picture shows.

Why would you include the GA snippet more than once, anyway?

If you're working on a legacy codebase like I am, it's really hard to find one place to include a script that will make it to every view of the website. This is because scripts/stylesheets are often not organized by an asset pipeline and included via templates - they are often hard-coded redundantly into various different views. In my case, different permutations of imperative PHP functions are used in all of the > 100 views of the website, each with hard-coded script elements. Therefore, to get full domain coverage, I have to include the GA snippet in each imperative function, which causes the snippet to get in multiple times.

Why is this even a problem?

This is a problem for two reasons. One, each script inserted to the DOM is another file the browser has to download which will slow down the page, especially on mobile and in poor network areas. It could be argued that the async attribute and the fact that the browser will cache after the first download, making this problem somewhat irrelevant to performance. Although it's still poor style to include duplicate scripts. The second, more potentially serious problem, is that the snippet sends a 'create' event, and a 'pageview' event. I'm not entirely sure what happens on Google's end, but I get nervous that with each duplicate snippet on the DOM these events will be duplicated as well. Result: wildly inaccurate GA page view data.

The hack

We can prevent this in a simple way - add an id to the script element, and check if an element with this id exists every time we want to create another script or send the 'create' and 'pageview' events. 

(function (i, s, o, g, r, a, m) {
    if (a) return;
    window['GoogleAnalyticsObject'] = 'ga';
    window['ga'] = window['ga'] || function () {
        (window['ga'].q = window['ga'].q || []).push(arguments)
    },
    window['ga'].l = 1 * new Date();
    a = document.createElement('script'),
    m = document.getElementsByTagName('script')[0];
    a.id = 'ga-script';
    a.async = 1;
    a.src = '//www.google-analytics.com/analytics.js';
    m.parentNode.insertBefore(a, m)

    window.ga('create', 'UA-xxxxxxxx-x', 'xxxxxx.com');
    window.ga('send', 'pageview');
})(window, document, 'script', '//www.google-analytics.com/analytics.js', 'ga', document.getElementById('ga-script'));
Specifically, we've moved our ga calls into the closure, making sure we explicitly call from window because we're no longer in the global namespace. To prevent script insertion and duplicate ga calls in one go, we simply pass in a as document.getElementById('ga-script') and make the very first line of the closure if (a) return;. If no element has the id ga-script, a == null and the function doesn't return. We also have to remember to give our new script element the id, by a.id = 'ga-script';.

And there you have it. I'm almost positive Google has a way of normalizing duplicate calls to its Analytics service, but just in case they don't, and for the sake of code cleanliness, I now use this hack in my web app.