AMP Pages Need to Have Canonical Tags
AMP pages without a canonical tag run the risk of not being considered to be valid, as well as creating duplicate content issues.
Canonicalising to Redirects Can Increase Time Taken to Index Preferred Version
Canonicalising to redirects can increase the time taken for Google to decide which version to index, especially if these pages aren’t identical. Google also looks at other signals such as internal linking and sitemap files to make this decision.
You Can See Which Canonical Google Has Chosen in the New Search Console
In the new Search Console you can select individual URLs which takes you to the info query showing you the canonical that Google has chosen.
Canonicals Are Chosen by Google Using XML Sitemap URLs
XML sitemap URLs are used to help inform Google’s decision on which URL is chosen to be the canonical.
Canonicalization For Filter Results Pages Isn’t Recommended
Canonicalization shouldn’t be used for filter pages. This is because canonical tags can be ignored and filter pages aren’t always the same as they have different types of results.
Incorrect Google Caching Suggests a Different Canonical Has Been Chosen
If the cached content from a different page is displayed on a page, this could be because Google has determined that the two pages are duplicates and has canonicalised one to the other.
Mobile Pages Must be Canonicalised to their Desktop Equivalents
If the desktop and mobile versions of a page both appear it means that Google doesn’t understand the connection between them. Make sure the mobile page has a canonical tag to the desktop page.
Rel Canonical Isn’t Required For Paginated Pages
If pagination is added to a page with rel next and rel previous it isn�۪t necessary to add a rel canonical to the page itself but it’s good practice.
Content on Canonicalised Paginated Pages Will Be Lost
Unique content and links on canonicalised paginated pages will be lost if Google accepts the canonical directive.