Checking if animation has already happened in AS3.

[jkell]

There are two ways to handle this:
1) You could just remove the event listener for the RollOver.
function manageMouseOver(event:MouseEvent):void{
gotoAndPlay("2");
button_mc.removeEventListener(MouseEvent.ROLL_OVER, manageMouseOver, false, 0, true);
}

2) You can create a variable and have a conditional statement
var isRolledOver:Boolean = false;

function manageMouseOver(event:MouseEvent):void{
if(isRolledOver==false){
gotoAndPlay("2");
}
isRolledOver = true;
}