You don’t HAVE to back up the Duplicati databases. It can be rebuilt from back-end storage. That being said, I do back up that database on my larger systems. Idea being I can avoid a possibly lengthy database rebuild.
To do this I use a second, separate backup job. It has in its selection list just the database from the primary backup job. This second backup is triggered automatically when the primary backup completes successfully (I use a post-backup script and duplicati-client to do this).