Multiple, mismatched canonical tags
This means that the URL in question has a canonical element specified in multiple locations (either in the HTML, in the HTTP header, or in a combination of both), and that the canonical URLs specified are not the same.
Why is this important?
If search engines encounter conflicting canonical URLs, they will ignore the canonical instruction entirely. This could result in the indexing of pages which you do not wish to be indexed, such as duplicate content.
What does the Hint check?
This Hint will trigger for any internal URL which contains multiple canonical link elements, where the canonical URL does not match. This may be one canonical found in the HTML and one in the HTTP header, or it may be two in the HTML, for instance.
Note: if this Hint triggers, it will also trigger the Hint:
Multiple canonical tags.
Examples that trigger this Hint:
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it had multiple canonicals defined, where the canonicals do not match;
Two different canonical tags in the <head>:
OR one canonical tag in the <head>:
AND a different canonical in the HTTP header
How do you resolve this issue?
The first task is to determine which is the correct canonical URL, and then ensure that this is the only one that is referenced.
Following on from this, you will also need to select one method of defining canonicals, and only use that method. If this is in the HTML, ensure that on all page templates the canonical is only being assigned once - reconfigure or remove offending plugins if necessary. You may need to seek developer input to determine which is easier to manage for each specific website.