You sync Contacts 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. It can also happen with MobileMe if you sync with MobileMe from your iPhone and also use iTunes to sync with your Mac or PC — especially if you set up MobileMe first.
Learn the details of fixing dupes when syncing Contacts on your iPhone in “iPhone Tip: The Trouble with Doubles (When Syncing Contacts)” in my blog, Bove’s Blips.
