title: Exception in thread "JavaFX Application Thread" duplicate children added
body: <p>I'm working in a application using JavaFx, I want to duplicate my composant in a my <code>StackPane</code> and I have an error/exception.</p>
  
  <pre><code>final Button file = new Button("fichier");
  file.setGraphic(new ImageView("/images/file.png"));
  final Image fichier_img = new Image("/images/file.png");
  final StackPane paint = new StackPane();
  
   ile.setOnAction(new EventHandler&lt;ActionEvent&gt;() {
     public void handle(ActionEvent e) {
          File fichier = new File();
          paint.getChildren().addAll( mes_images[0]);
         //fichier.setPositionX(me.getSceneX());
      }
  });
   paint.addEventHandler(MouseEvent.MOUSE_CLICKED,
      new EventHandler&lt;MouseEvent&gt;() {
         public void handle(MouseEvent e) {
              if (e.getButton() == MouseButton.SECONDARY)  
                  menu_gauche.show(paint, e.getScreenX(), e.getScreenY());
          }
  });
  </code></pre>
  
  <p>and Here is my error message: </p>
  
  <blockquote>
    <p>Exception in thread "JavaFX Application Thread"
    java.lang.IllegalArgumentException: Children: duplicate children
    added: parent = StackPane@658d001a at
    javafx.scene.Parent$2.onProposedChange(Unknown Source) at
    com.sun.javafx.collections.VetoableListDecorator.add(Unknown Source)</p>
  </blockquote>