aiming with animation

i want to have animation of aiming. i wrote my own script for it and its not working properly. every time i hold on the the right mouse button, the animation keep getting played over and over. is there a way to fix that?

here is the script.

var PlayAim = false;

function Update(){

 if(Input.GetButton("Fire2")){

 gameObject.animation["Aim"].speed= 6;

     animation.Play("Aim");

 }

 if(!Input.GetButton("Fire2")){

 gameObject.animation["Aim"].speed = -6;

 }

}