If you have recently added an additional Google/iCloud/Exchange Account to your FullContact account, and you are suddenly seeing duplicates in your Native contacts application, then this is likely because you have multiple sources contributing to your Contacts app.
You will need to make sure that your Contacts application is only receiving contacts from FullContact (via CardDav), from iCloud, or Google. The important thing is that you have only one of your syncing sources enabled with access to Contacts. If you had previously been using Google/iCloud as a source, you will need to disable these sources to ensure that you do not see duplicates.
For iOS devices:
Open your iPhone's Contacts app -> click on Groups in the upper-left-hand corner -> check only one contact source (either All Google, All iCloud, or All FullContact).
Also, please make sure that your device has been properly configured to use iCloud, Google, or CardDav. You can learn more about this here:
For Android Devices:
You will need to go into your Device Settings and then go to the settings for your Device Contacts application. Here, you should see Contacts to Display. Please select only ONE Google account to display in your Contact application. If you allow more than one account to show, then you will absolutely see duplicates.