You can also perform an export of Outlook information into a .pst, checking the "do not export duplicates" box. Wipe the device, then import your backup from that .pst back into the information store, again checking the "do not import duplicates" for good measure.

This will clean the dupes, and when you resync the device it will wirelessly import the now clean data into the unit.

Keeping in mind, that sometimes duplicates exist because records truly do have something different in them