When duplicating a quiz, and then changing the questions in the duplicated quiz, the origin quiz questions are also changed to what the duplicate quiz questions were changed to.

Steps to replicate: 

1. Create a quiz and add any qtypes (I've used true or false & Muli-choice),
2. Duplicate origin quiz and rename the duplicate (quiz 2),
3. Enter the newly duplicated quiz and modify the text of a question and save,
4. Go back to the origin quiz,
5. Note the origin quiz has now been updated with the duplicate quizzes question that was modified in in step 3.

I've replicated this on https://sandbox.moodledemo.net/

Versions I've replicated this on;

Moodle 3.9.2 (Build: 20200914)

Open LMS 3.8 (Build: 20200813)

Moodle 3.5 Stable (Build: 20200730)