Are one supposed to be able to mount more times on the same mount-point ?

Playing a little with my system I had a testdisk mounted on /mnt,
in another window I did a make in /usr/src/distrib/i386/floppies.
Part of the Make here mounts /dev/vnd0a on /mnt, and now all hell
broke lose :-(
Is this just me being thoughtless or shouldn't there be some checking.
(Is it possible to check for this ?)
The man pages don't say anything about this situation, and I must admit
not having tried it before.

Also the man page for mount_ffs says:
    The mount_ufs form of the command is meant for backward compatibility on-
    ly.  Fast File Systems should no longer be listed as type ``ufs'' in
    fstab(5) and instead should be listed as type ``ffs''.

but the result from mount is:
    bash# mount
    /dev/sd0a on / type ufs (local)
    /dev/sd1e on /Xe type ufs (local)
    /dev/sd1f on /port type ufs (local)
    /dev/sd2a on /NetBSD type ufs (local)
    /dev/sd2b on /usr/usenet type ufs (local)
    kernfs on /kern type kernfs (local)
    /dev/sd4e on /mnt type ufs (asynchronous, local)

regards
kim