Why does this part of code run twice?

I am making a basic inventory with stacking and for some reason whenever i click on the item it subtracts 2 instead of just 1. Can anyone help me? Thank you

 void OnGUI () 
     {
         GUI.DrawTexture(windowRect, inventoryBackgroundTexture);
 
         for (int x=0; x<across; x++)
         {
             for (int y=0; y<down; y++)
             {
                 Rect buttonRect = new Rect(_offset*2+buttonSize*x, _offset*2.5f+buttonSize*y, buttonSize, buttonSize);
 
                 if (inventory[x, y] != null)
                 {
                     if (inventory[x, y].itemCount > 0)
                     {
                         GUI.DrawTexture(buttonRect, inventory[x, y].texture);
 
                         Vector2 mousePos = Input.mousePosition;
                         mousePos.y = Screen.height-Input.mousePosition.y;
 
                         if (buttonRect.Contains(mousePos))
                         {
                             if (Input.GetButtonDown("Fire1"))
                             {
                                 SubtractItem(x, y); *** This part runs 2 times ***
                             }
 
                             GUI.DrawTexture(buttonRect, hoverTexture);
                         }
 
                         if (inventory[x, y] != null)
                         {
                             GUI.Label(buttonRect, inventory[x, y].itemCount.ToString(), itemCountStyle);
                         }
                     }
                 }else{
                     GUI.DrawTexture(buttonRect, blankTexture);
                 }
             }
         }
     }