SEO Canonization is making your URL standard and SEO friendly, that is, through URL the page will be described what it is all about. And as I said, URL should be "standard"- one URL format or structure per page. As what I have known, your backlinks will be affected if the URLs are not standard.
If canonical issue found in your site then it harmful for your site,,,,use 301 redirection to remove canonical issue..
There’s two ways to view most sites, that’s the www and non-www version. I won’t go into the full history of www (mainly because I don’t know it in detail), it’s enough to know when the first websites went online they were put in the folder www on the server (World Wide Web) and to access them you had to use the URL structure www.domain.com and it basically became the standard way to access a site via a browser. But, it’s not the only way, the true ‘location’ of the files are under domain.com (the www is in fact a sub-domain, you can have a completely separate site at www). This has led to problems with search engines like Google indexing both versions resulting in duplicate content and sharing link benefit problems. Some webmasters link to the www version and others the non-www, search engine spiders follow the links and spider the site twice. The major search engines have the ability to determine the www and non-www versions are the same and combine the results (passing all the benefit to just one version), but occasionally it fails and we see both versions indexed, we call this a www/non-www canonical issue. This is bad for the site owner since link benefit (PR/anchor text) is shared over two sites (making it harder to gain competitive SERPs). To check if you have a www/non-www canonical issue perform a site: search in Google- site:http://domain.tld The above search will find all pages indexed under the domain including the www and non-www versions. If you have a small site look through what is indexed, if you find both www and non-www pages indexed you have a canonical problem. For larger sites the next search will only find the www version- site:http://www.domain.tld If there’s differences you might have a canonical problem. To be safe use the .htaccess code listed below as then you’ll never have to worry about www and non-www canonical problems again. 301 Redirects Fixes the www/non-www Canonical Issue Fortunately a small amount of code within your .htaccess file located at root can solve the canonical problem. What this code does is 301 redirect one form of the site to the other, it’s like you setup an unlimited number of the simple 301 redirects I listed earlier on the fly. If you use the www version use this code RewriteEngine On RewriteCond %{HTTP_HOST} !^www\.seo-consultant-services\.co.uk$ RewriteRule (.*) http://www.seo-consultant-services.co.uk/$1 [R=301,L] The code above will 301 redirect the non-www version to the www version. Obviously replace seo-consultant-services and co.uk with your domain name/tld. Download the 301 redirect code in a pre-made .htaccess file and edit to your needs. If you use the non-www version use this code RewriteEngine On RewriteCond %{HTTP_HOST} !^seo-consultant-services\.co.uk$ RewriteRule (.*) http://seo-consultant-services.co.uk/$1 [R=301,L] The code above will 301 redirect the www version to the non-www version. Obviously replace seo-consultant-services and co.uk with your domain name/tld.
Canonization helps to remove duplicacy in search engine.SEO Canonization is very essential for cms like drupal,wordpress and many more
Usually when people say there are "canonical issues" it means there are duplicate pages. Different URLs with the exact same stuff on each URL. The way these issues are solved is to use canonical tags, pick the page you want to rank, then add rel=canonical tags to all the other duplicate pages, pointing them to the one you picked.
Canonical is the best process of presenting the best url to search engines. when you have 2 urls for same page, you should add canonical tag for duplicate page. in most of the website they have 2 urls for home page. .com and .com/index.html. it would be better to add canonical tag on index.html or else remove that page.