I am using the crypt encrypted backend with the sftp backend backing up to rsync.net, to backup a computer.
I now want to backup a second computer to the same backend. The second computer duplicates many of the files on the first. rsync.net uses ZFS internally and allegedly has deduplication turned on so hopefully the duplicated files will not count against my quota.
Does the crypt backend always encrypt the same file the same way, regardless of the file path? The two computers are running different operating systems, different file systems, and have different folder paths, but the actual files are identical and have the same names, and crypt is using the same password and salt for both.
No - each time it is encrypted it will be different as it has a random seed at the start, even if you are using the same password.
It has to be like this otherwise the cipher would be vulnerable to an attack.
You would work around though. Assuming you are backing up to a directory called A for the first computer and B for the second. What you could do is copy A to B before you sync the second computer. It will then delete all the files that aren't a match but keeping all the ones which are.
If you wanted to do this on a regular basis you could investigate the --copy-dest flag or the --compare-dest flag which you'd use to server side copy or ignore files in A when you were doing the sync for B.