GetMouseButtonDown detected twice

Heho,

guessing by your variable names, I assume you've running more than one of that scripts at the same time, thus making all instanced throwing the MouseDown event. To only call the MouseDown event on one particular object, make a raycast on click and compare the hit.collider.name with the GameObject's gameObject.name property.

Hope that helps

edit: Yeah, after reading over it again I see you have to change the code this way:

 var debugCnt : int = 0;
 var shouldAdd : boolean = false;
 
 var normalCostWood = 10;
 var normalCostStone = 20;
 var normalCostFire = 0;
 
 var fireCostWood = 20;
 var fireCostStone = 5;
 var fireCostFire = 2;
 
 function Start()
 {
     end = false;
     while(true)
     {
         if(end)
         {
             if(clone != null)
             {
                 Destroy(clone);
             }
             break;
         }
         var ray = Camera.main.ScreenPointToRay (Input.mousePosition);
         var hit : RaycastHit;
         if (Physics.Raycast (ray, hit, 1000, mask))
         {
             hitPoint = hit.point;
             if(!end)
             {
                 clone.transform.position = Vector3(hitPoint.x, 20, hitPoint.z);
             }
             if(!clone.GetComponent(canPlace).available)
             {
                 if(tower.transform.name == "TowerID")
                 {
                     if(Input.GetMouseButtonDown(0))
                     {
                         debugCnt++;
                         Debug.Log(debugCnt);
                         var wood = statsManager.GetComponent(stats).wood;
                         var stone = statsManager.GetComponent(stats).stone;
                         var fire = statsManager.GetComponent(stats).fire;
                         if(tower.transform.tag == "normalTower")
                         {
                             if(wood >= normalCostWood && stone >= normalCostStone && fire >= normalCostFire)
                             {
                                 statsManager.GetComponent(stats).wood -= normalCostWood;
                                 statsManager.GetComponent(stats).stone -= normalCostStone;
                                 statsManager.GetComponent(stats).fire -= normalCostFire;
                                 
                                 debugClone = Instantiate(tower, Vector3(hitPoint.x, 20, hitPoint.z), Quaternion.identity);
                                 debugClone.GetComponent(towerHP).statsManager = statsManager;
                             }
                         }
                         if(tower.transform.tag == "fireTower")
                         {
                             if(wood >= fireCostWood && stone >= fireCostStone && fire >= fireCostFire)
                             {
                                 statsManager.GetComponent(stats).wood -= fireCostWood;
                                 statsManager.GetComponent(stats).stone -= fireCostStone;
                                 statsManager.GetComponent(stats).fire -= fireCostFire;
                                 debugClone = Instantiate(tower, Vector3(hitPoint.x, 20, hitPoint.z), Quaternion.identity);
                                 debugClone.GetComponent(towerHP).statsManager = statsManager;
                             }
                         }
                     }
                 }
             }
         }
         yield;
     }
 }
 
 function OnEnable()
 {
     clone = Instantiate(dummyTower, Vector3(0,0,0), Quaternion.identity);
     Start();
 }

In this code, you have to make the tower differntiation by using of tags, as the you have to assign a unique name to each tower when instantiating to compare it to the hit target.