Canonical URLs help solve duplicate content issues.
Pages with duplicate or similar content may confuse search engines, as they wouldn't know which of the 2 similar pages deserve to rank.
This is where canonical URLs come in handy.
Canonical URLs prevent duplicate content issues by pointing all your pages with similar content to the “preferred” version.
Canonical link is placed in the header of the document and looks as follows:
<link rel="canonical" href="https://ahrefs.com/blog/seo-metrics/>
Our bots crawl the entire page, and, thus, can find canonical links in the header.
In case you are wondering, why would anybody link to your domain via “rel=canonical” tag, the answer is pretty simple. If some site syndicates content from your website (in a proper way) they must declare that the content they publish is a copy by declaring your original page as a canonical version.
Now, in Site Explorer, canonical links are marked with a respective tag and shows that some other domain points to your page as to a canonical version of the content:
You will be happy to find out that you will benefit from all the backlinks the duplicate page gains.