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Google Search Console has a featured where you can specify your preferred domain but you don't need it, says Google's John Mueller if you have 301 redirects in place.
The preferred domain is the one that you would liked used to index your site's pages (sometimes this is referred to as the canonical domain). Links may point to your site using both the www and non-www versions of the URL (for instance, http://www.example.com and http://example.com). The preferred domain is the version that you want used for your site in the search results.
Google's John Mueller said in a Google Webmaster Help thread that if you have a 301 redirect in place, then you really don't need it. He wrote, "As long as you have your redirects set up, there's no need to set a preferred version for your domain -- it'll happen automatically."
The interesting part is that the documentation says that after you set the preferred domain, you should make sure to set up the redirects. It reads, "Note: Once you've set your preferred domain, you may want to use a 301 redirect to redirect traffic from your non-preferred domain, so that other search engines and visitors know which version you prefer."
Forum discussion at Google Webmaster Help.
