Click on GameObject leads to multiple call

Hey guys facing a strange issue.While detecting the click on gameobjects using Raycasting,sometimes the code gets executed multiple times. Here is the code

 void Update () 
 {
     if (Input.GetMouseButton(0)) 
        {
         Ray ray;
         RaycastHit raycast;
         ray=Camera.mainCamera.ScreenPointToRay(Input.mousePosition);
         if(Physics.Raycast(ray,out raycast))
         {
             switch( raycast.collider.name)
             {
                 case "First_Sphere":
                     GameObject go = (GameObject)Instantiate(Resources.Load("1ValenceBondElementPrefab"),new Vector3(0,0,0),Quaternion.identity); 
                 break;
                     
             }        
         }
     }    
 }

What leads to this multiple calling and why sometimes it leads to multiple calls and sometimes it is executed properly