When working in Outlook, and especially when syncing with mobile devices, it can happen that you have duplicate items created. 10 items isn't a problem, but 100 or more can get tedious to clean manually.
There is software available to help you sort through the duplicates quickly. I have listed four of them below. They range in price from $15 to $30 and differ slightly in operational modes and user interface. They all have trial/demo versions though so you can get a feel for which one you like best.
But don't forget - always make a backup of yout Outlook .PST file before running any of these programs.

anti-dupe.com
nomoredupes.com
outlook.duplicate-remover.com
duplicatekiller.4team.biz

 Update: I just heard about a free program that may work for you: http://www.vaita.com/ODIR.asp