Actually in cases like your example above its more an issue of an ambiguous URL rather than actual duplicate content.
The thing to do in the example above is to choose which version of your site you want (with www or without) to always use, and then set your server accordingly. In Apache this means using your .htaccess file.
If you decide to always display www (my preferred way) then this should be in your .htaccess:
<ifmodule mod_rewrite.c="">RewriteEngine on</ifmodule>
RewriteCond %{HTTP_HOST} !^www. [NC]
RewriteRule ^ http://www.%{HTTP_HOST}%{REQUEST_URI} [L,R=301]
If you want your URLS to not use www:
<ifmodule mod_rewrite.c="">RewriteEngine on</ifmodule>
RewriteCond %{HTTP_HOST} ^www.(.+)$ [NC]
RewriteRule ^ http://%1%{REQUEST_URI} [L,R=301]