Many, including myself, have ended up with duplicate entries in Outlook calendar/contacts and after syncing with the PDA, the duplicates appear on your phone. After a while (after two-way syncing) you may end up with triples / quadruplicates / can't-even-spell-the-next-level.
Here is a site describing potential solutions to the problem http://www.sperrysoftware.com/Outlook-Duplicates.asp. They even sell add-ins that will eliminate the duplicates for you.
Has anyone tried these or similar add-ins?