What is Canonical URL?
A canonical tag, also known as a "rel canonical" is a way of letting search engines know which specific URL is the master page. This is necessary if you have multiple variations of the same page on the site.
By using a canonical tag, you can prevent potential "duplicate" content problems.
A canonical tag or canonical URL 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.
It can be used for reacquiring pages for the home page.
If you have a canonical tag in website backend coding, it is supposed to tell Google that the website is having unique content. If we don't define canonical tags for our websites, Google will consider the websites are with duplicated content.
A canonical URL refers to an HTML link element, with the attribute of rel="canonical" , found in the <head> element of your webpage. It specifies to search engines your preferred URL.
Canonical issue like your website open with non www and www and /index.html but all have same content, so you should use 301 redirect other pages to www(.)domain(.)com