ofxSceneManager duplicate scenes

As I said, you must not create your gui in setup() of your ofxScene, it is called each time the scene is changed.

sceneManager.changeScene(0);

-> calls setup()

You should create an init() or initGUI() method and call it when you create your scene

FirstScene* firstScene = new FirstScene;
firstScene->setSceneDuration(0.5, 1.5, 0.5);

firstScene->initGUI()