This means that the URL in question has a canonical element pointing at another internal URL. The URL in question can therefore be described as 'canonicalized.'
If Page A is canonicalized to Page B, it is explicitly marking Page B as it's canonical. This acts as a strong signal to search engines that these two pages should be considered duplicate, and that Page B should be prioritised.
It is important to note that a canonical is not a robots directive, so search engines may choose to ignore a designated canonical if they disagree with it. However, assuming that search engines honour the canonical, this means that Page A may become deindexed, and Page B will consolidate link signals (i.e. if Page A has some external links pointing at it, Page B will get the value for these links).
See Google's directions on defining canonicals to consolidate duplicate content.
This Hint will trigger for any internal URL which contains a canonical link element (either in the HTML or in the HTTP header) which specifies another internal URL as the canonical URL. An internal URL is defined as being on the same subdomain.
Consider the URL: https://example.com/page-a
The Hint would trigger for this URL if it had either of the following;
Canonical link in the <head> to another internal URL:
<link rel="canonical" href="https://example.com/page-b" />
OR HTTP Header canonical link to another internal URL:
HTTP/... 200 OK
...
Link: <https://example.com/page-b>; rel="canonical"
Similarly, consider the URL: https://example.com/page-a?colour=red
The Hint would trigger for this URL if its canonical URL was the unparametrised version of the URL (or indeed any other internal URL);
Canonical link in the <head> to the unparametrised URL:
<link rel="canonical" href="https://example.com/page-a" />
OR HTTP Header canonical link to the unparametrised URL:
HTTP/... 200 OK
...
Link: <https://example.com/page-a>; rel="canonical"
This Hint is an 'Insight', which means there isn't necessarily any action that needs to be taken - the Hint is intended to alert your attention to something, rather than flagging up an issue that needs fixing.
Canonicals implemented in this manner are usually deliberate, so they are flagged in Sitebulb in order to draw your attention to the URLs that have been canonicalized, so you can double check that they have been implemented correctly.
While Sitebulb can check if a canonical tag is valid, it has no way of knowing for sure that Page B should be the canonical URL for Page A.
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