Example: Index page: www.domainname.com/articles
if you type with www. or with out www, you are taken to that index page.
However, if you’d like to read an article with out the www, you are taken back to the index page.
Example: if you type domainname.com/articles/baseball_coaching.html with out www, you taken back to the index page.
Would this be looked at as duplicate content?
According to Google, as long as you choose one and stick with it, there should be no adverse impact on your page rank.
If you use both www. and direct, the search engine may see one of the sites as a mirror, therefore decreasing the page rank of both or, worst case, de-listing you.
You can add a simple .htaccess redirect to ensure that one goes to the other without a problem; also, using Google’s sitemaster tools you can specify which one to use in the sitemap tool.