It would be very useful to have the option -one-file-system, which shall prevent the backup process from copying files and folders from another file system. It’s a quite popular option among other tools 
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What other tools is this popular in? How is the utility supposed to tell which filesystem to include files from?
E.g. Borgbackup, rsync or tar. The filesystem where the root of the repository is used and if a subdirectory is in another FS, then that boundary is not crossed. An exception would be, that a subdir would explicitly be specified to be backed up.
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Count my vote in.
I’m looking at using duplicacy to backup a “/var/lib/docker/volumes” directory which contains local and NFS mounted files. As this thread suggests, an option to stay on the local file system would make the filters file much easier to maintain.
For now, I’ll manually update the filters file. If I put together a script to identify and update a list of directories to exclude, I’ll post it here.
On linux, macOS and other BSD systems, these is syscall stat, fstat and lstat that returns the ID of the device.
On windows, since the path starts with the descriptor for the disk, I think it is fairly easy to implement it.
I just got badly burned by this by mounting my NAS in a place that was inside where duplicacy was backing up, so it tried to back up my entire NAS to B2, which costs non-zero money.
As previously mentioned, both rsync and tar have a --one-file-system option; this seems like a fairly basic feature to me, honestly, I was really surprised duplicacy didn’t have it.