The &id changes on each visit, and is used to identify the session. Google considers the entire URL including the &id= portion when indexing a page. When the spider returns to the site, a new code will be generated by your server for the &id field, causing Google to see a new duplicate of the page each time it visits.

Session variables such as &id should be stored in cookies so that they can remain invisible to the search engines. It will also make it easier for others to link to your content, because they won't accidentally include the &id portion. Also, I should mention that having tracking code in the URL rather than in a cookie can represent a security threat if your shopping cart uses that variable.

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The best way to learn anything, is to question everything.