title: GWT onPreviewNativeEvent runs 2 times when key is pressed
body: <p>I use the event to show a message when I press the ESC key, but it shows the alert twice. Where is the mistake?</p>
  
  <pre><code>Event.addNativePreviewHandler(new Event.NativePreviewHandler() {
      @Override
      public void onPreviewNativeEvent(Event.NativePreviewEvent event) {
          NativeEvent nativeEvent = event.getNativeEvent();
              if( nativeEvent.getKeyCode() == 27){
              Windows.alert("HELLO");
          }
          }
      });
  </code></pre>