Howto Fix “XFS: Filesystem has duplicate UUID”
So I just forgot to unmount
borg mount and disconnected the USB drive and it got stuck there. The solution: restart.
But if the UUID is really the same, these guys https://computingforgeeks.com/how-to-solve-xfs-filesystem-has-duplicate-uuid-error/ advise:
mount -o rw,nouuid /dev/sdaX /mnt xfs_admin -U generate /dev/sdaX