parallel/simultaneous portinstall ?

[Ronald F. Guilmette]

One question about portupgrade has been nagging me, in the back of my
mind, for some time now.

Assume for the moment that on some given system, the root user has two
terminal sessions open, i.e. either two instances of xterm or else two
console sessions.

Also assume that there exist three ports, `A', `B', and `C' such that
both port `A' and port `B' both depend on port `C'.

Assume further that port `C' has not yet been installed.

My question is just this:  Is it possible that Something Bad might
happen if, in one terminal session, the root user does "portinstall A"
and if, which that instance of portinstall is still running, he then
immediately switches to his other terminal session and then does
"portinstall B" ?

Another way of stating my question is this:  Might anything bad ever
happen if there are two simultaneous, or nearly simultaneous attempts
to build & install any given single package (e.g. port `C' in my
example above) ?

(I am concerned about the possibility that the two simultaneous attempts
to build and install the same single package might possibly trip over
one another, resulting either in a mangled build tree or else a mangled
installed port, or possibly both.)