This is the story. I have found that when I sync my iphone to itunes, with both the 'sync with google contacts' and 'sync with yahoo contacts' boxes "checked"....after the sync is complete, I will end up with 20+ duplicate contacts in my address book It gets worse.....I then check my google contacts, same thing! Yep, your probably asking, 'what about your yahoo contacts?'....YES! same disaster!
Don't believe me? Try it yourself, I have tried disabling yahoo, and then sync....same problem. I then tried disabling google/gmail contacts sync...same problem. It's unbelievable....you can have 96 contacts in your iphone, PERFECTLY arranged... then plug in to sync to itunes (thinking you will be saving your new and edited contacts you've entered throughout the day) to sync to your address book (which in turn should then sync to google/yahoo), and after your sync, check your iphone and some (or all) of your NEW contacts are gone!!
The way I understand it, if you have either 'sync with google/yahoo contacts' enabled and you sync your iphone to itunes, do not think your 'overwriting' your address book on your mac, your not. The only method I can see using is, every week or so...delete your old contacts and create and import a CSV file for yahoo and gmail off address book. If someone has a way to actually 'push' new OR edited contacts to gmail or yahoo, please enlighten me!! At this point, I would send you a paypal bonus!