A quick search returned these hits
http://tinyurl.com/abwk9
http://www.sperrysoftware.com/Outloo...Eliminator.asp
Unfortunately both the free script and the buyware assume you have all
of your mail items in the same folder.
I'd think you could do this by checking for duplicate
ConversationIndex values, which you can do with items in different
folders and even different PST's. That might involve the brute-force
method of creating a database of all ConversationIndex values and then
reporting on where the duplicates are so that you can choose which
ones to delete.
I'd write this but I'd have to sell it for about $25 per copy, sorry.