Duplicates in contacts in iphone

On your Mac, open the Address Book and goto Card → Look for Duplicates.

You'll be prompted with something like this:

After merging the contacts in the Address Book you should choose to replace the contacts on the iPhone. This option can be found under the iPhone in the iTunes sidebar.

Best to not sync your contacts with iTunes and a cable if you're using Mobile me and/or iCloud. The duplicates are probably because you're looking at what's on your computer combined with what's on iCloud.

Once you choose to sync with iCloud turn off contacts and calendar sync through iTunes. This was true in the "old" Mobile Me days and it's still true with iCloud.