From Google WebmasterCentral:
http://googlewebmastercentral.blogsp...ed-by-url.html
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When user and/or tracking information is stored through URL parameters, duplicate content can arise because the same page is accessible through numerous URLs.
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And what to do about it?
Quote:
1. Removing unnecessary URL parameters -- keep the URL as clean as possible.
2. Submitting a Sitemap with the canonical (i.e. representative) version of each URL. While we can't guarantee that our algorithms will display the Sitemap's URL in search results, it's helpful to indicate the canonical preference.
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