Go through the contacts in your Mac's Address Book and delete those not current or duplicated.
If you have set-up your Mac to sync to MobileMe and you have set-up your MobileMe account on your iPhone to sync mail, contacts, calendars and bookmarks from MobileMe, DO NOT select contacts to sync from your Mac in the iTunes iPhone Info pane. This will cause duplicates.
Your Mac's Address Book contacts will sync to the MobileMe server and be pushed to your iPhone. Contacts added on the iPhone will be synced to the MobileMe server and synced to your Mac's Address Book. All three "devices" will stay in sync with each other.
The only reason to select the Mac's Address Book contacts in the iTunes iPhone Info pane would be if you sync your iPhone to a Mac that is not synced to your MobileMe account, and you want to add those contacts to your iPhone.
If you haven't read these yet, read them now:
http://www.apple.com/mobileme/setup/index.html
http://www.apple.com/mobileme/setup/iphone_ipod.html