Canonical URL: how to tell Google about duplicate pages

Google Webmaster Central Blog: “Carpe diem on any duplicate content worries: we (Google) now support a format that allows you to publicly specify your preferred version of a URL. If your site has identical or vastly similar content that’s accessible through multiple URLs, this format provides you with more control over the URL returned in search results. It also helps to make sure that properties such as link popularity are consolidated to your preferred version.”