Google, Microsoft, Yahoo all together announced a new tag to resolve the duplicate content problem.
Syntax is pretty simple. Lets say you have this ugly url
Code:
http://www.example.com/page.html?sid=asdf314159265
and you want to tell search engines that the url is actually
Code:
http://www.example.com/page.html
To accomplish that you just put code like this in the HEAD part of the document:
Code:
<link rel="canonical" href="http://example.com/page.html"/>
Read more
Learn about the Canonical Link Element in 5 minutes
And here is the video from Matt Cutts: