Multiple 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).
Why is this important?
It is considered best practice to only specify canonicals once on any given URL. This is because doing it multiple times makes the configuration more open to human error.
Imagine you add an SEO plugin to your site which allows you to set canonicals, and by default it sets self-referential canonicals. At a later date, you add another plugin which also allows you to set canonicals, and by default it also sets self-referential canonicals.
At this point, there would be nothing 'wrong', as all the canonicals are in agreement. However, if in the future you wanted to change a canonical for a particular page, and went into the plugin configuration and set this up, there exists the potential that you may completely forget that the other plugin is also setting canonicals, and so you would end up with one plugin still setting the self-referential canonical, and the other plugin with the new canonical URL.
The net result would be mismatched canonicals, which would cause search engines to ignore the canonical instruction entirely (which would then trigger the Hint: Multiple, mismatched canonical tags).
You can avoid such catastrophic futures by only specifying canonicals once.
What does the Hint check?
This Hint will trigger for any internal URL which contains multiple canonical link elements. This may be one in the HTML and one in the HTTP header, or it may be two in the HTML, for instance. This Hint will trigger even if the canonical URLs are the same.
Examples that trigger this Hint:
Consider the URL: https://example.com/
The Hint would trigger for this URL if it had multiple canonicals defined, whether or not the canonical URLs were the same;
Two canonical tags in the <head>:
OR one canonical tag in the <head>:
AND one canonical in the HTTP header
How do you resolve this issue?
Simply 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.