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:

Google, Yahoo and Microsoft have detailed explanations of how they work.

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:

Plus, Yoast already posted plugins to support this for WordPress, Magento and Drupal.

Forum discussion Google Webmaster Help, Cre8asite Forums, WebmasterWorld and Sphinn.