Wait dont ever shave it,,,,it will come back thicker,,, I don't know what it is called when a woman has a mustache, but i have seen it before on a female,,,and a beard too,,,i'ts called very hairy face. lol
It is a hormone problem, and a girl with a mustache should always try waxing or some other form of female hair removal because shaving it is always a bad idea. I know a girl that has this problem. It appears as if she does try to shave and that makes it worse.