You get this if you use the “Google Drive” button here:
https://duplicati-oauth-handler.appspot.com
The “Google Docs” button gives access to the entire drive.
There is a little text on the page that describes it:
If you have previously used the Google Docs backend for Duplicati, and want to continue the backup, you must use this option. Using the Google Docs button will grant access to all files in your Google Drive, where the Google Drive button will only grant access to the files created by Duplicati itself.