ZydoSEO - 3:44 pm on Jan 14, 2013 (gmt 0)
You need to choose either www or non-www as your canonical ("preferred") form of URLs for your site and 301 redirect all non-canonical page requests to the canonical form. You also need to consider whether to show or hide default document names (index.html, index.php, default.asp, etc) when referencing folders w/ default documents as well as whether to show or hide the trailing slash when referencing folders w/ default documents. All of these and more (eg. query string parameters) can create canonicalization issues and lead to split page rank and duplicate content.