Update 19.9.: Fixing the bug requires both a version upgrade of the STACK plugin and a core modification to Moodle. Based on our testing, updating STACK alone does not fully resolve the duplication issue. The fix and the STACK update will be most likely done on the maintenance evening on 23 October.
Teacher using STACK questions: Since the beginning of July, there has been a bug in the STACK question type, as a result of which all STACK questions in the course question bank are duplicated when a new copy of any quiz activity is created. Therefore, avoid copying quizzes within your course and create new quizzes with the Add Activity function.
Copying quizzes from the old course to the new one works normally and does not need to be avoided.
The time to fix the STACK error is still open. We will inform you about it later.
If you don't use STACK math questions, you can copy quizzes without worrying about any side effects.