Recently, it has been talked a lot about Google and duplicate content as well as Google Canonical problems.That is, when you have your site accessible both under your_domain.com and www.your_domain.com. To avoid such problems you can use the following lines in your .htaccess file to force only the www version of your web site:
RewriteEngine on
RewriteCond %{HTTP_HOST} !^www.your_domain.com$
RewriteRule ^(.*)$ http://www.your_domain.com/$1 [R=301]
Please, note that the .htaccess should be located in the web site main folder.
This will redirect all requests to the non-www version of your site to the www version using 301 Permanent redirect which will make the search engines to index your site only using the www.your_domain.com URL. In this way you will avoid a duplicate content penalty.