Multiple null mounts with same parameters

[Charlotte Koch <charlotte%NetBSD.org@localhost>]

Hi,

I discovered that there's nothing preventing anyone from making multiple
null mounts from the same source directory *to* the same mountpoint.

    # mount -t null /src /dest
    # mount -t null /src /dest
    # mount -t null /src /dest
    ...

You can umount /dest multiple times, too, and it just works. That's
pretty neat; I suppose the "stackable layer" design of the file system
doesn't prevent this sort of interaction from happening. But to me, it
still feels wrong to permit a mount with the *same* source and the
*same* destination multiple times.

Does it make sense to prevent the user from creating a null mount whose
parameters match an existing null mount?

Charlotte