If you want to create multiple copies of the same quiz, you need to copy the latest copy each time: if you have a Canvas quiz named "Quiz 1", for example, you can duplicate it by copying it, and then you will have two identical quizzes named "Quiz 1". If you duplicate the original quiz a second time (even if you had renamed/edited the first copy!), it will overwrite the first copy. In order to create more copies without overwriting the first copy, you need to rename your copy to e.g. "Quiz 1 copy" so you can distinguish between the original and the first copy, and then to create another copy of the same quiz, you instead need to copy the first copy, "Quiz 1 copy", and so on. Just make sure that you never duplicate the same quiz more than once, and it should be fine.