I wish we had this option as well. I have all my data on freenas and duplicati runs as a docker. I map everything using NFS. Currently I’m using rclone as a jail to satisfy the effort of backing up my google drive to local in case google ever …i dunno stops working. Rclone is really great for backing up between clouds, local to cloud, cloud to local. I would like to do this in duplicati just to centralize my backup efforts. That being said, and looking at the configuration file for a backup job, it SEEMS like backing up from cloud to cloud, or cloud to local should not be so complicated and may already be supported just no commands or ui description exists. Feature request 
In theory, if you can get your cloud to present itself as a mount point / drive letter then Duplicati will likely work just fine. However default settings do assume a local / speedy source so may not be optimal for a cloud-as-source setup.
This may also apply to rclone and similar tools, but when Duplicati detects a change in a file it looks at the entire file to determine (and back up) only blocks with changes. If you have a large cloud file with even a small change that could cause a lot of download bandwidth…
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