If I delete synced Google contacts on my iPhone, does that affect my google contact?
If you're only seeing the duplicates on your iPhone, they're most likely in different Groups. You can check this by opening Contacts on your phone and tapping the Groups button (screenshot below). If you see a group called iPhone, those are stored exclusively on your phone and deleting them will not affect your Google contacts. You can uncheck the Google group to show (and ensure you only delete) contacts from your phone. Deleting contacts from the Google group will affect your Google contacts.