Space Invaders style game bug.

Hello,I'm making a space invaders style game. But there's a bug in my game and I can't find a way to fix it. I my game, I have the 'Asteroid' scene, where you shoot and collect points, and a 'shop' screen, where you can buy upgrades. My problem is when you press the 'Next Wave' button in the shop, my game starts the wave, but 2 times. Could some one please tell my what I'm doing wrong? This is my script for the 'Asteroid' scene:

 var IsSpawning : boolean = false;
 var CanLoadShop : boolean = true;
 var CanLoadLoss : boolean = true;
 
 var Asteroid : Transform;
 
 var WaveNumber = 0;
 
 static var Score : int = 0;
 static var Lives : int = 3;
 static var HighScore : int = 0;
 static var EnemyCounter = 0;
 
 var test : int = 3;
 function Awake(){
     DontDestroyOnLoad(transform.gameObject);
 }
 
 function OnLevelWasLoaded(level : int){
     if(level == 3){ //The asteroid screen is 3
         Debug.Log("Level 3 was loaded");
         UpdateWave();
         CanLoadShop = true;
         CanLoadLoss = true;
     }
     if(level == 4){ //The shop screen is 4
         CanLoadShop = false;
         CanLoadLoss = true;
     }
 }
 
 function Update(){
     if(Lives <= 0 && CanLoadLoss){
         if(Score > HighScore){
             HighScore = Score;
         }
         Application.LoadLevel("ScreenLoss");
         CanLoadLoss = false;
     }
     if(EnemyCounter == 0 && !IsSpawning && CanLoadShop){
         Application.LoadLevel("ScreenShop");
     }
 }
 
 function UpdateWave(){
     WaveNumber++;
     WaveFunction(WaveNumber);
     Debug.Log("Wave: " + WaveNumber);
 }
 
 function WaveFunction(Wave : int){
     IsSpawning = true;
     
     var EnemysInThisRound = Wave * 4;
     
     Debug.Log(EnemysInThisRound + "Enemys");
     for(var i = 0 ; i < EnemysInThisRound; i++){
         var position = Vector3(Random.Range(-6, 6), 12, 0);
         Instantiate(Asteroid, position, Quaternion.identity);
         yield WaitForSeconds(Random.Range(1, 1.5));
         EnemyCounter++;
     }
     IsSpawning = false;
     
     Debug.Log("Stopped");
 }

Here's my shopscript:

 function OnGUI(){
     if(GUI.Button(Rect(267,0,77,33),"Next Wave")){
         Application.LoadLevel("Asteroid");
         Debug.Log("Pressed next wave");
     }
 }

If it could help, here's my debug log:

Thank you - Nelis