Specifying your canonical preferences to avoid duplicate content problems

The ability to arrive at the same content on a web site through a number of different URLs has been a problem for many web sites. Different sort parameters, category navigation or tracking parameters can give the semblance of duplicate content. The search engines have difficulty reconciling these different URLs as actually pointing to the same content. Instead they see each URL as pointing to a different location for the identical content - not great.

Now Google, MSN and Yahoo have agreed on a new header tag that allows you to specify your canonical preference, page by page, regardless of the specific URL that actually got you there.

You can read a good post on the subject of this new canonical header tag from Google here. Yahoo (adding more arrows to your quiver) and MSN (partnering to help solve duplicate content issues) also have plenty of guidance on the subject.