root@phschafft-x2:/tmp/mount-tests# ls -laFhi
total 20K
3940458 drwxr-xr-x  3 root root 4.0K Oct 16 15:39 ./
3932161 drwxrwxrwt 63 root root  12K Oct 16 15:39 ../
3932567 -rw-r--r--  1 root root    0 Oct 16 15:38 a
3932653 -rw-r--r--  1 root root    0 Oct 16 15:38 b
3940459 drwxr-xr-x  2 root root 4.0K Oct 16 15:39 c/
root@phschafft-x2:/tmp/mount-tests# mount --bind a b
root@phschafft-x2:/tmp/mount-tests# ls -laFhi
total 16K
3940458 drwxr-xr-x  2 root root 4.0K Oct 16 15:38 ./
3932161 drwxrwxrwt 63 root root  12K Oct 16 15:38 ../
3932567 -rw-r--r--  1 root root    0 Oct 16 15:38 a
3932567 -rw-r--r--  1 root root    0 Oct 16 15:38 b
3940459 drwxr-xr-x  2 root root 4.0K Oct 16 15:39 c/
root@phschafft-x2:/tmp/mount-tests# umount b
root@phschafft-x2:/tmp/mount-tests# mount --bind c b
mount: /tmp/mount-tests/b: mount point is not a directory.
       dmesg(1) may have more information after failed mount system call.