Help with removing and duplicating movieclip

i have this function that everytime i click the screen a box appears.

function box () {
_root.onMouseDown = function(){
movie = _root.attachMovie(“mcBox”, “mcBox”, _root.getNextHighestDepth());
movie._x = _xmouse;
movie._y = _ymouse;
}
}

how can i make it so i only have one box on the screen at a time
like this

if (there is a box on the screen) {
function = false
} else { continue function; }

i cant figure out how to do that in actionscript.