Unity 3D C# - GameObject Alredy Exists

This is my Code

     void  Update (){
 //display the diestance and direction of where the player is looking
         Ray ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         Debug.DrawRay(ray.origin, ray.direction * MaxDistance, Color.green);
 
         RaycastHit hitLook;
             //Distance = hitLook.distance;
         if (Physics.Raycast (transform.position, ray.direction * MaxDistance, out hitLook)) {
             if (Distance < MaxDistance)
             {
                 if(hitLook.transform.gameObject.tag == "AllowedSelection")
                 {
                     if (myCubeLook)
                     {
                         //Destroy(myCubeLook.gameObject);
                     }
                     else
                     {
                         spawnLocation = new Vector3(hitLook.transform.position.x , hitLook.transform.position.y, hitLook.transform.position.z);
                         Instantiate(myCubeLook, spawnLocation, Quaternion.identity);
                     }
 
                     //myCubeLook.transform.position = hitLook.transform.position;
                 }
             }
         }

problem is i can't simply destory the object, What I want to happen is like in minecraft I look at an area and there is like a shadow version of the object I am trying to place.