17.5.

mount --bind mount-point mount-point
mount --make-unbindable mount-point

# mount --bind mount-point mount-point
# mount --make-unbindable mount-point

mount --bind mount-point duplicate-dir

# mount --bind mount-point duplicate-dir

mount: wrong fs type, bad option, bad superblock on mount-point,
missing codepage or helper program, or other error
In some cases useful info is found in syslog - try
dmesg | tail  or so