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Every AMP document needs to have a link referencing the "canonical" version of that document. We'll learn more about what canonical pages are and different approaches to canonical linking in the Making your page discoverable step of this tutorial.

Canonical linking in regular HTML pages is a common technique for declaring which page should be considered the preferred page when multiple pages include the same content.

You don't have to name your AMP files as .amp.html. In fact, AMP files can have any extension you want. It's common to see publishers differentiating AMP pages from their canonical versions by using parameters in the url. For example: http://publisher.com/article.html?amp.