The method setTrackerUrl is registered more than once in "_paq" variable

[igortregub]

I've faced the same issue as It was described in #15

Preconditions

1. Magento 2.2.3
2. henhed/module-piwik 2.0.0
3. Google Chrome

Steps to reproduce

1. open dev console
2. add some product to the Shopping Cart
3. go to the checkout page
4. see errors on the dev console

Expected result

1. there mustn't be any errors on the dev console

Actual result

1. there are js errors:
   

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if this issue you cannot reproduce, refresh your checkout page few times

It's happening because on the checkout page code
https://github.com/henkelund/magento2-henhed-piwik/blob/master/view/frontend/templates/piwik.phtml#L36-L49
runs in paralel with code
https://github.com/henkelund/magento2-henhed-piwik/blob/master/view/frontend/web/js/tracker.js#L298-L303
as result tracker.js tries to inject piwik.js file second time.

My PR adds checking if such script already exists on this page: #27

[henkelund]

Hi @igortregub!

I tried, but I couldn't reproduce this issue in my environment. But I can see how it could happen so I've merged your PR and released v2.0.1.

Thanks for your contribution

[igortregub]

Thank you :)