Canonical loop
This means that the URL in question has a canonical element whereby the canonical URL is actually canonicalized back to the original URL (e.g. Page A is canonicalized to Page B, which is then canonicalized back to Page A).
Why is this important?
The point of a canonical is to explicitly and unambiguously indicate a preferred URL. However in this situation, the canonical instructions are sending conflicting messages to the search engines - which typically means they will ignore the canonicals entirely and try to make their own decisions about what 'the canonical URL' should be.
The net result is that it could lead to incorrect URLs being indexed, resulting in duplicate content issues.
What does the Hint check?
This Hint will trigger for any internal URL which contains a canonical link element for a URL that canonicalizes back to the original URL.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it had a canonical URL pointing at another internal page,
where this canonical URL https://example/com/page-b has a canonical pointing back at the original URL:
The above example shows canonicals defined in the HTML, but this would also trigger if the canonicals were defined using HTTP headers (or some combination of the two methods).
How do you resolve this issue?
This is a scenario where the canonical configuration is completely broken. You will first need to establish if the correct canonical for Page A should actually be Page B, and if so adjust the canonical URL on Page B so that it is self-referential.
Conversely, if the correct canonical for Page B should actually be Page A, then adjust the canonical URL on Page A so that it is self-referential.
Issues like this will typically need to be resolved on a case by case basis.