When you are experiencing duplicate contacts or calendar items on a BlackBerry device, always try the following workaround before wiping or reactivating the device (which is always a risk, because some important personal settings could be lost). 

Note: this workaround will wipe only the contacts or calendar items and resync them.

  1. On the Home Screen, tap Contacts (or Calendar).
  2. Press the menu button, tap Options in the Address Book or Calendar menu.
  3. In the Options menu, type rset. (There is no field to type in rset command, type it on your keypad when you get to Options)
  4. Select Yes in all the confirmation windows that are displayed.