Yesterday, Google, Yahoo and Microsoft announced together a new way to handle internal duplicate content issues with a new "canonical" header tag. Vanessa Fox does an excellent job explaining what it is all about in her piece at Search Engine Land.
So for all duplicate pages, you insert this tag in the header elements of those pages, specifying the main URL. The tag looks like this:
<link rel="canonical" href="http://www.example.com/true-url.html" />
Three main things:
(1) This works only internally, not across domains. (2) Treat this like you would a 301 redirect, so be careful (3) Search engines consider this a "hint" and do not have to abide by it (just yet)
Outside of that, there is good recaps on this at Techmeme.
We have a ton of Q&A on this from our live coverage of the Ask the Search Engines panel from SMX West. I am sure your questions are answered in that panel or in the discussions below.
This tag can be confusing, because it is new. But after webmasters begin to understand where, if and how to use it, they are more likely to love it.
JohnMu said in a forum post:
Here are some examples where this could be used: - Web-shops (mutliple URLs depending on how you got to a page) - Sites that work with Session-IDs within the URL - Ad-tracking URLs (eg using AdWords + Analytics) - Affiliate tracking URLs - News sites with multiple URLs per article - Forums with multiple URLs per thread/page (eg "&highlight=", etc)
Plus, Yoast already posted plugins to support this for Wordpress, Magento and Drupal.