Go inside your phonebook, press menu, select View.
If for instance your google account has the same contacts as your sim/phone, and more than 1 of the tickboxes in the View menu are selected, it can give the appearance that multiple contacts are being shown, when in reality they are just coming from different sources.
If you log onto your Gmail (probably best/quicker to do on a pc) and go to contacts, then select each set of duplicates and clik on merge. Think this is somewhere at the top right of the contacts section.
Don't have access to Gmail at the minute since i'm at work, so can't give full detail but this is how i got rid of my duplicate contacts.
I don't believe the contacts that are stored on your phone will automatically sync back to Gmail contacts. Maybe there's an easier way of doing this, but this is what I did to initially set up:
Connected HTC Hero to my PC, and used HTC sync to store all contacts on Microsoft Outlook.
Deleted ALL contacts from phone.
Exported all contacts from Outlook to a csv.
Imported csv to GMail.
Merged duplicates in GMail.
Imported contacts from Google on HTC
Now all my contacts are unique, and saved on Google only.
I have lot of duplicate contacts on my phone however there seems to be an app that allow you to merge duplicate contacts and also linked your contacts on facebook to your phone contacts, please does anyone remember this app or what do I have to do to merge all my contacts into one. it is very frustrating right now to have so many of the same contacts. Thanks