Some weirdness with recursive instantiation.

I have an object which when clicked, will instantiate another prefab of itself via Resources.Load & Instantiate. The problem is that subsequent clicks to the object (or instantiated clones) clones all of them, quickly multiplying.

It feels like one of two things are happening (or possibly both) 1) The mouse raycast is somehow hitting the colliders of multiple objects (Can I "consume" the ray?) 2) The prefab instantiating itself is somehow causing recursion (or is affecting the actual prefab)

What can I do to enable an object to instantiate itself without this multiplying effect?

Code is beneath using UnityEngine; using System.Collections;

 public class CubeBuilder : MonoBehaviour {
     
     public Camera camera;
     public GameObject prefabToInstance;
     
     // Use this for initialization
     void Start () {
         camera = Camera.main.camera;
     }
     
     // Update is called once per frame
     void Update () {
         if (Input.GetMouseButtonUp(0))
         {
             Ray t = camera.ScreenPointToRay(Input.mousePosition);
             RaycastHit rh = new RaycastHit();
             
             if (Physics.Raycast(t,out rh))
             {                
                 Instantiate(prefabToInstance,transform.position+new Vector3(0,0.1f,0),transform.rotation);
             }    
         }
     }
 }

This code is placed on a cube, with a collider (size < 0.1f), cube is made into prefab, and prefab is linked to itself (GameObject reference)