Find this tip and many more in Tony’s Tips for iPhone User’s Manual, a $2.99 iPhone app by Tony Bove.
A lot of my friends have had troubles with doubles after jumping into iPhone 3.0 and syncing with Exchange or MobileMe. A typical scenario: you try to sync Contacts to not only synchronize with the Exchange Server at work, but also with your Contacts folder on your PC — which may be already synced to Exchange. This results in duplicates of every contact (with some in triplicate and quadruplicate, if that’s a word).
You can sync with both your “local” PC folder and Exchange — the two sets of contacts are separated into Groups on the iPhone. You can select only one of the two Groups, rather than All, when accessing contacts on your iPhone (the other Group would always be there, but with dupes of the first Group). But this can be inconvenient, and if you also sync your PC folder of Contacts with Exchange, you could end up with dupes in both Groups.
This scenario can also happen with MobileMe if you sync with MobileMe from your iPhone and also use iTunes to sync with your Address Book on your Mac, or Outlook on a PC — especially if you set up MobileMe first. If you use iTunes syncing first, then set up MobileMe syncing, you get the choice to merge your Contacts, and you shouldn’t have dupes.
But what if you already “got duped”?
You can turn off syncing your Contacts in iTunes (if you are still syncing them from iTunes). Next, turn off syncing of your MobileMe or Exchange Contacts (not Calendars, Bookmarks, or Mail, just Contacts) on your iPhone. When you do, you are given the choice to remove the existing data from your iPhone, which is a good thing and you should do it. You can then resync, starting fresh with the Contacts from the MobileMe or Exchange server.
For all this (and more details), see Resyncing iPhone Contacts to Fix Duplicates or Corrupt Entries in Tony’s Free Tips (adapted from my iPhone app, Tony’s Tips for iPhone Users Manual).
That should fix your trouble with doubles, even trouble with triples. Quadruples? You got problems.