Like rel=”nofollow” URL tag, a new rel Tag (canonical) has been introduced and currently being supported by major search engine to tell the search engines that the specific URL is the duplicate.
<link rel="canonical" href="http://www.example.com/printed-version.html">
This tag makes the webmasters free from relying with server end 301 redirect management, since this URL tag will be work as same as 301 redirects. And this can help retain the Authority of the home or any other page by disregarding pages that support session IDs, affiliate versions, printed, pdf versions, etc. that create duplicates of the original page.
More FAQ on this is on SEOMoz.
See what Matt Cutts, the Web Spam Head in Google has to say about it: