With or Without WWWs
If your site is accessible with and without the www prefix, Google will see this as duplicate content.
The problem is that you have no control over which version Google will decide to index. If, for example, you site winds up getting indexed without the www but all your links point to your domain with the www, you links will effectively be useless.
To combat the issue between www and no www,you simply need to put a rewrite rule in your .htaccess file. Drupal has already place the appropriate code in this file so all you need to do is uncomment the following statements in your default .htaccess file.
# (http://example.com/... will be redirected to http://www.example.com/...)
# adapt and uncomment the following:
RewriteCond %{HTTP_HOST} ^example\.com$ [NC]
RewriteRule ^(.*)$ http://www.example.com/$1 [L,R=301]
Don't forget to change the example name to your own website. It's a good idea to verify that the rewrite rule works by typing in your URL without the www prefix and you should see it redirect.
