Removing duplicate reliably and automatically is tricky.
Robert Jacobson has a script called itunes
removeduplicates at:
http://home.comcast.net/~teridon73/itunesscripts/
You might want to check this out, but you need to be sure you have a good backup in case you don't like the result. You can look at the script to get an idea what it does if you understand that kind of thing.
P.S. You haven't given you operating system, the above applies to Windows. If you have a Mac you may find something useful at;
http://dougscripts.com/itunes/
Message was edited by: polydorus