Canonical Tag - Avoid Duplicate Content Issues

One of Google's blogs recently stated that they have introduced a new 'canonical tag' for the head section of websites to help webmasters specify the preferred version of a url.

Many websites, particuarly large e-commerce sites with multiple search options, encounter problems where accessing the same product via different variable choices leads to identical or extremely similar content being indexed by search engines. It can also be caused by a site attributing a session id to each site visit.

To avoid being punished for duplicate content issues, you can use the new
link rel="canonical" tag in the head section of any web pages that could throw up duplicates.

Eg add:

link rel="canonical" href="http://www.example.com/product.php?item="blue-widgets"

inside the head section of the duplicate content URLs eg:

http://www.example.com/product.php?item=blue-widgets&category=industrial-tools

http://www.example.com/product.php?item=blue-widgets&trackingid=1234&sessionid=5678

This will let Google know that the duplicate content urls should not be indexed, but rather just the primary url.

This page:
http://googlewebmastercentral.blogspot.com/2009/02/specify-your-canonical.html

of the Google Webmaster Central blog gives more information on this new tag.