If syncing continues to cause these headaches, you might try resetting sync history. Here's how I'd do it:
1. Make sure all your contacts, appointments, and tasks
on your Mac are updated & correct.
2. Launch the iSync app, go to Prefs, and click the Reset Sync History... button. Proceed.
3. From the HotSync Manager, set the Contacts, DateBook, and Tasks/ToDo conduits to overwrite the handheld.
4. HotSync. If you're asked to merge information, don't: overwrite/replace the Treo's information with the Mac's information.
If dups still appear, try brute force:
1. Download & install
Filez. (Typically this requires a hot sync, but disable all except the Install conduit.)
2. Repeat steps 1-3 above.
3. Launch Filez, tap View and Edit Files, tap the triangle next to Internal, and check (by tapping) the following files: CalendarDB-PDat, ContactsDB-PAdd, and TasksDB-PTod. Delete them.
4. HotSync.
Let us know successes or failures. Good luck!
-K
P.S. If you want to easily delete duped entries should they occur, try
this app.