Find out if duplicates are on your phone and / or also in you google gmail account on a computer. If they are on the computer, you could delete all your contacts on your phone (Settings - Manage applications - choose all applications to list - find contacts (there are two on my phone) listed. Open each and delete the data stored on your phone. Your phone will now show no contacts.
Go to your pc and log into your Gmail account. Click contacts on the left. Along the top and to the right is a 'more' drop down dialog box. Click 'find and merge duplicates'. You'll be given a box showing the duplicates and can choose which ones you want to merge. Click that and it will clean up your duplicates.
Go back to your phone and go into accounts, click on your gmail account and sync your contacts (don't know if you use usb, or what. I use Companionlink). Should be fine and good to go.