text: Title: A little enhancement for win32_hooks.py method _process_kbd_msg_type
  Body: I propose a little enhancement for win32_hooks.py method _process_kbd_msg_type.
  While debugging keyboard hooks, I noticed that the self.pressed_key array often contains the same key several times. I propose to add change the line
  
          if event_type == 'key down':
  
  in this method to
  
          if event_type == 'key down' and current_key not in self.pressed_keys:
  
  This keeps the list shorter.
  