Google search console provides variety of features to analyze our website performance. A canonical tag is an onsite SEO aspect that tells search engines the page is having similar content to another page and that which one is the master page.
The syntax for this code is as below:
<link rel=”canonical” href=”here comes URL of similar content page”>
This tag is placed in the code of the respective page (either page from the similar ones). Apparently there is no harm of having this notification in your google search console however, if you unnecessarily add pages of the similar content would affect your search ranking. Hence you’d be deprived of potential traffic that was meant to conversions.
Google search engine also identifies similar content pages and stores/indexes only the parent version of the page. That’s the reason you see few pages are indexed and rest duplicate aren’t.
Here is example of how you see this notification in your google search console.
How to Fix?
Just after the report seen in the image above, you’ll find reasons why your pages weren’t indexed. Simply select on the link “Alternate page with proper canonical tag” and it’ll get you to the list of page with canonical tags/duplicate content.
You may proceed and fix the issue by having unique content pages with no duplicate throughout your website pages. Once done you may validate in the google search control and you’ll see no further notifications any such.
The image below is a reference where you can see “Validate Fix” button that you can click once your resolve the issue.
There can be multiple reasons behind why your pages are not indexed, like:
Alternate page with proper canonical tag
Duplicate where google choose different url to index than the user
Page with redirect
Excluded by the “noindex” tag