I have not used that tool, but from the name it sounds like it makes a drive image. You need Administrator privileges for this as well, but they have perhaps built it in such a way that the part that does the actual image is an elevated process that is then invoked from your user account.
Duplicati does not currently support that mode of operating, but you can set up Duplicati to run as a Windows Service, which will allow you to use VSS snaphots, so you can read all files.
There is a guide for setting up Duplicati as a Windows Service here:
https://docs.duplicati.com/detailed-descriptions/using-duplicati-with-windows#running-the-server-as-a-windows-service