Backup UrBackup to external storage was your UrBackup forum version, and both are quiet so far… Sometimes very specific questions don’t get answers, and some searching is necessary. One piece:
UrBackup forum has prior discussions of how to external-backup a UrBackup, but generally the same concern arises. One person says that rsync -H is hard link aware. Some suggest methods that don’t require walking a file tree (and maybe getting confused by links), e.g. try block-oriented tools instead, perhaps even BTRFS or ZFS replication, but all of that depends on OS. Duplicati starts with file scan, meaning it will have to work very hard if you have trees of duplicated files that are hardlinked together.