I do use rsync, robocopy and rclone to backup the backups + backup directory snapshotting with versioning. But due to serious Duplicati reliability issues I would recommend running separate backup sets, instead of copying the possibly non-restorable backups to multiple destinations. At least in that case there’s possibility that you can restore from another source, when the restore fails with one set. - And do test your backups with full restore. The duplicati test option unfortunately isn’t reliable.
Edit: Repair getting rewritten is great news. Let’s hope it finally ends the extremely serious non-restorable backup issues. I’m sure the issue isn’t technically big, but the ramifications of non-restorable backups are just huge.
I can be sure that I can restore that backup using Duplicati on a third computer without any problems?
No you can’t, unless you test it often. And if you do, you’ll find out that it works mostly, but at times it doesn’t. See my post linked above.
Edit:
Database recreate - It can be slow, sometimes finds issues, but it exists.
Yeah, it can take weeks and fail after that.