That is your problem. Quizzes and questions are separate animals. When you duplicate the quiz, it contains a single question which lives in the quiz bank. So, when you edit the question, you edit it in both quizzes. (This is actually a good feature once you understand it!)
What you need to do is remove the question from the duplicated quiz and create a new one.
What I normally recommend is that you create a quiz with no questions in it - then duplicate it so that you keep all the settings that you want. Then, add questions to each individual copy - that avoids the possibility of doing what you did ... and what all of us have done at one point, I am sure!!