Exponential Instantiation

The original ball fires then I click once. one ball. click twice, 2. then four and so on.

public class Reload : MonoBehaviour { public Rigidbody Cannonball; public Transform clip;

 void Start()
 {
     
 }

 // Update is called once per frame
 void Update()
 {
     if (Input.GetButtonDown("Fire1"))
     {
         Instantiate(Cannonball, clip.position, clip.rotation);
     }
     
 }

}

Why is this happening? Easiest Fix?