从数组中获取随机对象，如果对象相同，则获取一个新对象

I have made a jokes application, where the user generates a joke and the joke will display in a UILabel. However I am trying to randomise the jokes show, but I do not want to show the same joke twice. Please could you tell me how I could do this. I am using the code below to try and do that, but it seems that it is not working.I have made a jokes application, where the user