Very popular tool for searching duplicate items (seem serial number ). He's name is Dupefinder.
This running under DOS and searching SQL database. Actually he looking for illegal serial numbers. Muserwer know every item by his serial number. so when any stupid player try to duplicate item - this fake stuff has different serial number.
Finally, and of story.. find this user and delete his account. Simple. So.. make this in every day if you care about your muserwer.

Config is in dupefinder.ini file. If you chang pass and login in SQL, then change this seem in this file. If not.. leave it. :).

So, just start up dupefinder.exe and wait for results.
That's all folks.