Hi,
this is expected behavior. When you duplicate a quiz, you don't create a duplicate of the questions but you're using the same questions (or something like that). As a consequence, any edit in the questions of the new quiz changes the questions wherever they are.
If you want to re-use a question and modify it, you have to use the Question Bank. First, create/edit the questions, then create a quiz, and finally add the corresponding questions to that quiz.