Robert_Charlton - 6:10 am on Oct 30, 2011 (gmt 0)

proboscis - the Hot Topics area [webmasterworld.com], which is always pinned to the top of this forum's index page, has a section on duplicate content, with good reference threads discussing a large variety of issues that can cause a page to be returned under multiple urls. It can take a while to get through them all, but it's well worth the effort.