mounting the same device several times

[nj18]

I discovered that I can mount /cdrom1 as many times as I want:
nohost# mount /cdrom1
nohost# df
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/ad0s1a    253678   104038   129346    45%    /
devfs               1        1        0   100%    /dev
/dev/ad0s1e    253678    30714   202670    13%    /tmp
/dev/ad0s1f  36603172 17667326 16007594    52%    /usr
/dev/ad0s1d    253678    78342   155042    34%    /var
linprocfs           4        4        0   100%    /usr/compat/linux/proc
/dev/acd1      716352   716352        0   100%    /cdrom1
/dev/acd1      716352   716352        0   100%    /cdrom1
nohost# mount /cdrom1
nohost# df
Filesystem  1K-blocks     Used    Avail Capacity  Mounted on
/dev/ad0s1a    253678   104038   129346    45%    /
devfs               1        1        0   100%    /dev
/dev/ad0s1e    253678    30714   202670    13%    /tmp
/dev/ad0s1f  36603172 17667326 16007594    52%    /usr
/dev/ad0s1d    253678    78342   155042    34%    /var
linprocfs           4        4        0   100%    /usr/compat/linux/proc
/dev/acd1      716352   716352        0   100%    /cdrom1
/dev/acd1      716352   716352        0   100%    /cdrom1
/dev/acd1      716352   716352        0   100%    /cdrom1
nohost#