Why is the report telling I have duplicate content for 'www' and No subdomain?

[cpisano]

i am getting duplicate content for most of my pages. when i look into in your reports the 'www' and 'no subdomian' are the culprit. How can I resolve this as the www.domain.com/page and domain.com/page are the same page

[Matt-Williamson]

Hi Chris

You need to setup a 301 redirect from your none www.domain to your www.domain or vice versa depending on your preference. You should also make sure you have set your preferred version in Google Webmaster Tools.

Take a look here for SEOMoz's guide on redirection -

http://www.seomoz.org/learn-seo/redirection