1021 duplicate function

programming flash
1

im using this code:
stop();
dna10_btn.addEventListener(MouseEvent.MOUSE_DOWN,mouseDownHandler2);
function mouseDownHandler2(event:MouseEvent):void
{
nextFrame();
}

throughout my frames in flash cs4 but it keeps coming up with 1021: duplicate function and i know i have to change the function name but how do i change the function name?