steveb - 12:16 am on Nov 25, 2008 (gmt 0)

If you don't have a redirect set up properly, all it takes to get duplicate content trouble like this is for someone to link to one of your URLs with a querystring attached, and then for google to screw up and list the query string URL instead of the correct one.

They usually get it right, but they sometimes make a mistake and list the wrong one.