AMP Page URL has canonical URL which is canonicalized
This means that the URL in question is an AMP Page URL, and contains a canonical tag pointing to a canonicalized URL (i.e. the canonical URL is itself canonicalized to another URL).
Why is this important?
AMP HTML documents are required to have a canonical to the non-AMP equivalent (or a self-reference, if no equivalent exists). If this canonical is pointing to a URL that is in itself canonicalized, then something has been configured incorrectly.
The canonical tag is a mandatory element for AMP pages to be considered valid, and the canonical tag is supposed to point back at the original 'non-AMP' version of the page. If this page is itself canonicalized then it is not indexable, which sends conflicting and confusing messages to search engines - increasing the chances that the AMP page will not show up in search results.
What does the Hint check?
This Hint will trigger for any internal URL which references an AMP Page, where the AMP page defines a canonical URL which is canonicalized.
Examples that trigger this Hint
Consider the AMP Page URL: https://example.com/page-a/amp/
The Hint would trigger for this URL if it included a canonical URL,
where this canonical URL was itself also canonicalized:
How do you resolve this issue?
Fix the canonical on the AMP page so that it points back to the non-AMP version of the page, and ensure that the non-AMP version of the page has a self-referencing canonical. It should look like this:
For the URL: https://example.com/page-a/
This page defines a self-reference canonical and an AMP page:
Then the AMP page has a canonical pointing back at https://example.com/page-a/