In a course with 2 different categories of questions when you duplicate a quiz with randow questions on a specific category, if you change the category for a randow question of the duplicate quiz, it will change the category of the original quiz too.
Step by step instructions :
1. Create 2 question categories with at least one question, Cat1 and Cat2
2. Create a quiz, Quiz1, with one random question on Cat1 category
3. Duplicate Quiz1 into Quiz2
4. Change the random question category of Quiz2 from Cat1 to Cat2
5. Look at Quiz1, category of random question is now on Cat2