Official Google Webmaster Central Blog: Specify your canonical
... Carpe diem on any duplicate content worries: we 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 ... [full article
"explains canonical attribute which is used to google know the alternate url for any content." - mf
Thank you mf for bookmarking this article!