Instantiate prefab

Hi... i am new to unity ... ^_^ .. what i am trying to do is to create prefab on the spawn which i have positioned, when first prefab create , wait for 8 seconds then create rest of 4... but here what is happening ... its creating prefabs continously ... like first on on location 1 it creates hundreds of prefab :( ... then after 8 sec starts creating hundreds of on other locations..:"( .. what i want just create one one prefab on each location .. just once ... kindly help me out ...

 #pragma strict
 
     var timer : float = 0.0;
     var spawning : boolean = false;
     var prefab1 : Rigidbody;
     var prefab2 : Rigidbody;
     var prefab3 : Rigidbody;
     var prefab4 : Rigidbody;
     var prefab5 : Rigidbody;
 //    var prefab6 : Rigidbody;
     var spawn1 : Transform;
     var spawn2 : Transform;
     var spawn3 : Transform;
     var spawn4 : Transform;
     var spawn5 : Transform;
 //    var spawn6 : Transform;
     var flag=true;
      
     function Update () {
   
      if (flag){
       Spawn();
       }
   
     }
      
     function Spawn(){
      
      var location1 : Transform;
      var location2 : Transform;
      var location3 : Transform;
      var location4 : Transform;
      var location5 : Transform;
 //     var location6 : Transform;
      
           
       location1 = spawn1;
       location2 = spawn2;
       location3 = spawn3;
       location4 = spawn4;
       location5 = spawn5;
 
      var thingToMake1 : Rigidbody = Instantiate(prefab1, location1.position, location1.rotation);
      yield WaitForSeconds(8);
      var thingToMake2 : Rigidbody = Instantiate(prefab2, location2.position, location1.rotation);
      var thingToMake3 : Rigidbody = Instantiate(prefab3, location3.position, location1.rotation);
      var thingToMake4 : Rigidbody = Instantiate(prefab4, location4.position, location1.rotation);
      var thingToMake5 : Rigidbody = Instantiate(prefab5, location5.position, location1.rotation);
   flag=false;
     }