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A canonical page is the preferred version of a set of pages with highly similar content.
It's common for a site to have several pages listing the same set of products. For example, one page might display products sorted in alphabetical order, while other pages display the same products listed by price or by rating. For example:
http://www.example.com/product.php?item=swedish-fish&trackingid=1234567&sort=alpha&sessionid=5678asfasdfasfd
http://www.example.com/product.php?item=swedish-fish&trackingid=1234567&sort=price&sessionid=5678asfasdfasfd
Source :http://support.google.com/webmasters/bin/answer.py?hl=en&answer=139394
September 15, 2012, 8:40 am