A single click on a button produces a mouse event where getClickCount() returns 2. A double click returns 4, and so on.

Test program:

public class DeleteMe extends Application {
    public static void main(String[] args) {
        launch(args);
    }

    @Override
    public void start(Stage stage) throws Exception {
        final Label label = new Label();
        Button button = ButtonBuilder.create()
                .text("Click me")
                .onMouseClicked(new EventHandler<MouseEvent>() {
                    @Override public void handle(MouseEvent event) {
                        label.setText("Click " + event.getClickCount());
                    }
                })
                .build();
        Scene scene = new Scene(VBoxBuilder.create().spacing(10).children(label, button).build());
        StageBuilder.create().scene(scene).visible(true).applyTo(stage);
    }
}

The context is that we have a set of ToggleButtons that can be used to select a template. You can select either by clicking on one of the buttons and then on a Select button, or by double-clicking on the button. But in fact the double click shows up as a single click.