What can go wrong with Canonical Tags
Search engines will ignore your canonical tags in the following situations:
Content different on canonical and canonicalised URL:
Google may choose to ignore canonical tags if the canonical URL and the canonicalised URL are different.
Page missing a canonical tag:
All pages should contain a canonical tag to prevent any possible duplication, including on the canonical page.
Canonicalising to the wrong URL:
If the canonical URL is not similar enough to the canonicalised one, then Google will probably ignore it.
Multiple canonical tags:
If the canonical tags on the same page are different, then Google will ignore both.
Canonical loop:
A page canonicalises to a page that canonicalises back.
Unlinked canonical pages:
Most canonical URLs would probably be linked internally at least once because they are usually an important part of the site.
If a canonical URL is not linked directly it may indicate the canonical URL is wrong.
Redirecting canonical URL:
If the canonical URL redirects to another URL, then it can’t be a true canonical URL.
Broken canonical URL:
If the canonical URL isn’t a valid URL then Google will probably just ignore it but it will still waste time, which reduces crawling efficiency.
Empty canonical tag:
Canonical tag does not include a URL
Search engines will ignore your canonical tags in the following situations:
Content different on canonical and canonicalised URL:
Google may choose to ignore canonical tags if the canonical URL and the canonicalised URL are different.
Page missing a canonical tag:
All pages should contain a canonical tag to prevent any possible duplication, including on the canonical page.
Canonicalising to the wrong URL:
If the canonical URL is not similar enough to the canonicalised one, then Google will probably ignore it.
Multiple canonical tags:
If the canonical tags on the same page are different, then Google will ignore both.
Canonical loop:
A page canonicalises to a page that canonicalises back.
Unlinked canonical pages:
Most canonical URLs would probably be linked internally at least once because they are usually an important part of the site.
If a canonical URL is not linked directly it may indicate the canonical URL is wrong.
Redirecting canonical URL:
If the canonical URL redirects to another URL, then it can’t be a true canonical URL.
Broken canonical URL:
If the canonical URL isn’t a valid URL then Google will probably just ignore it but it will still waste time, which reduces crawling efficiency.
Empty canonical tag:
Canonical tag does not include a URL