Sorry to read of your frustrations with duplicating a quiz. That certainly sounds frustrating. I'm not too sure that I have an answer as to "why" this is happening...as it seems the steps you've been trying should be successful. By chance, do you have access to Canvas Commons? If so, another option for you to try would be to share the quiz to Canvas Commons. Then, import the quiz from Commons back to the course (changing the name of the quiz each time).