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).
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.
This Hint will trigger for any internal URL which references an AMP Page, where the AMP page defines a canonical URL which is canonicalized.
Consider the AMP Page URL: https://example.com/page-a/amp/
The Hint would trigger for this URL if it included a canonical URL,
<!doctype html>
<html amp>
<head>
<meta charset="utf-8">
<title>Sample document</title>
<link rel="canonical" href="https://example.com/page-b/" />
...
</head>
...
</html>
where this canonical URL was itself also canonicalized:
<link rel="canonical" href="https://example.com/page-c/" />
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:
<link rel="amphtml" href="https://example.com/page-a/amp/">
<link rel="canonical" href="https://example.com/page-a/" />
Then the AMP page has a canonical pointing back at https://example.com/page-a/
<link rel="canonical" href="https://example.com/page-a/" />
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