A canonical link element is an HTML element that helps webmasters prevent duplicate content issues by specifying the "canonical" or "preferred" version of a web page as part of search engine optimization. It is described in RFC 6596, which went live in April 2012.
What is the canonical issues in SEO?
A canonical issue arises when 301 redirects are not properly in place. This means that your website can be accessed by search engines from several different URLs. This means that search engines can then potentially index your site under different URLs, meaning that it will look like a site of duplicated content.
What is a canonical error?
A canonical tag (aka "rel canonical") is a way of telling search engines that a specific URL represents the master copy of a page. Using the canonical tag prevents problems caused by identical or "duplicate" content appearing on multiple URLs.