Once you've run a crawl of your site in the Site Audit tool, you might see a 'Non-canonical page specified as canonical one' warning in the report under the Internal pages menu: 

Canonical links is a sure-fire way of solving duplicate content issues.  

By placing rel canonical tag in the header of your HMTL you tell search engines which piece of content is the original and which one is a duplicate. 

For more info on canonical links feel free to check Google's guide.

What does "Non-canonical page specified as canonical one" warning implies?

It tells you that pages specified as canonical on other pages of your website have a "rel=canonical" link to a different page.

This creates a so-called "canonical chain" where page A links to page B that links to page C from their "rel=canonical" elements.

How to fix this? 

According to John Mueller, Google "can follow canonical chains, but we strongly recommend that you update links to point to a single canonical page to ensure optimal canonicalization results."

It basically means that all similar pages of your website should point to one page that you consider to be the authoritative page for all of those similar pages. 

Canonical chains may confuse search engine crawlers; misconfigured "rel=canonical" will be ignored.
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