the backups in borg and restic are not classical copies, each snapshot is not independent, it is a collection of hashed blocks. In a sense, time machine and hard links do the same: they reference the same bits across snapshots, saving space. for the bit rot / faulty sector or failed drive, you use redundancy with RAID or similar in your file system. if all else fails and your backup system burns in a fire, then you reach for your other backup. deduplication and redundancy solve different problems.