Update: Duplicate master V2.16 (Download link). (04/08/10)
The add-in works in MS Excel 2003 and 2007. It was originally distributed through the site – http://members.iinet.net.au/~brettdj which is offline these days.
You can however download the the duplicate master xls file and use it for free.
Edit: Thanks to ElmasnoJ the duplicate master has got a companion – the help file. You can now download the help file for using the excel add-in.
Edit (Feb 2, 2010): Dave has released the new version of the add-in, Duplicate Master Version 2.13, Please go to the comment for the download link.