In general, things that get backed up to your Google account will be data from Google apps (e.g., Google Calendar, Google Contacts, etc.), your system settings, and data from some (but not all) of your 3rd party apps -- it depends on whether the developer decided to implement that backup function. You can see more detail if you go to the Google Dashboard (myaccount.google.com/dashboard) and look under Android. It tells you which apps backup data on Google servers.
I'm pretty sure that locally saved Phone account contacts and calendar events (i.e., contacts and events saved only on the phone and not synced to any cloud account) and SMS messages don't get backed up to Google. I doubt that you'd get duplicates for data backed up on Google, but I've never tried it myself.