Specifying a preferred domain for your website helps with SEO and is very important. Unless you apply canonicalization you might be getting damage from SEO point of view. If you don’t specify a preferred domain, search engines may treat the www and non-www versions of the domain as separate references to separate pages which could expose your website to duplicate-content issues.
Specifying a preferred domain (canonicalization):
-On the Search Console Home page, click the site you want.
-Click the gear icon , and then click Site Settings.
-In the Preferred domain section, select the option you want.
You will need to verify ownership of both the www and non-www versions of your domain. But don’t worry. It is as easy as uploading an html file to your website and then clicking a link.
Note that specifying a preferred domain alone will not be enough. You also need to create a 301 redirect from www to non-www domain or vice versa. Check out http://big.info/2012/11/how-to-redirectforward-domain-to-sub.html for more info on 301 redirection.
Hope this helps.
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