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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
Canonical issues occurs when more than one form of url exist in the web such as if a website is accessible by both the version of website i.e. with www and without www than there exist the problem of link canonicalization. The best possible way to solve this issue is to redirect one form of URL to another form of URL. This will make the Google look both the form of url as a single will avoid content duplication and similar problem. You can solve it through .htaccess file.
A canonical issue arises when 301 redirects are not properly in place. This means that search engines can then potentially index you site under different URLs meaning that it will look like a site of duplicate content.
Canonical was created, used for the purpose of preventing the duplicate content of a Website. It will help the administrator inform the search engine of the original content, quality content on the Website.
Whenever content on a site can be found at multiple URLs, it should be canonicalized for search engines. This can be accomplished using a 301 redirect to the correct URL using the rel=canonical tag or, in some cases, using the Parameter Handling tool in Google Webmaster Central.