1. Create a new Quiz.
2. Add some questions to the Quiz, for example 10 Gapfill questions.
3. Duplicate the Quiz.
4. The questions in the Quiz will now appear twice in the question bank.

Another oddity:
If there is a parent category and child categories within it, e.g.
Parent A

• Child A
• Child B
• Child C

And a quiz contains only questions from the Child A category, if I duplicate such a quiz, the questions are duplicated in the Child B and Child C categories as well.

Question bank status before duplicating a quiz:

Question bank status after duplicating a quiz:

Categories before duplicating a quiz:

Categories after duplicating a quiz (which contains only Interactive bash tasks)