It is now a widely known fact that duplicate content is bad (with some exceptions such as BLOGS where it is expected). Yesterday on the Google Webmaster Central Blog, Maile Ohye posted some important notes on duplicate content directed at those who use multiple URL parameters on their site.

Essentially, variables placed in a URL for the purposes of tracking will result in duplicate content. This tracking can be in the form of affiliate ID’s, session ID’s, or any other form of tracking you may be implementing.

While the article does not address if there are possible penalties due to these duplicate URL’s it does point out that these longer URL’s can offset branding efforts and decrease the chance of your listing being selected by the searcher.

So how can you help search engines to understand your site? By removing any unnecessary URL parameters and submitting an XML sitemap with the clean version of each of URL will help, but your best bet is to permanently clean it up through the use of cookies. By 301 redirecting any inbound affiliate ID or tracking encoded links to the short clean URL and storing those affiliate and tracking variables in a cookie you can ensure that only one version of each URL is accessible on your site.

Duplicate content can come in many forms. Ross Dunn has published some articles which cover other aspects of duplicate content:

Most Common Reason for Dropped Rankings
What Determines Duplicate Content SPAM?
Does Article Syndication Amount to a Duplicate Content Penalty