Outlook duplicate message finder
If you move Outlook PSTs around from an old machine to a new one it's quite probable that you'll end up with duplicate messages and maybe contacts too.
There are numerous software packages to find and remove these but, s far as I know, only one free one, and that's Outlook Duplicate Items Remover 1.4.2. This is quite an 'old' package, and was most recently upated in January 2010,so the Authors really do keep ontop of any changes.
You can find it here - it's free and it just works. Highly recommended.