mount on top of a mount

peter:/mnt # ls test
notmounted
peter:/mnt # mount /dev/testvg/test test
peter:/mnt # ls test
lost+found  test
peter:/mnt # mount /dev/testvg/test2 test
peter:/mnt # ls test
lost+found  test2
peter:/mnt # mount | grep test
/dev/mapper/testvg-test on /mnt/test type ext4 (rw,relatime,data=ordered)
/dev/mapper/testvg-test2 on /mnt/test type ext4 (rw,relatime,data=ordered)
peter:/mnt # umount /dev/mapper/testvg-test2
peter:/mnt # ls test
lost+found  test
peter:/mnt # umount /dev/mapper/testvg-test
peter:/mnt # ls test
notmounted