Duplicate activity works using Backup and Restore. It automatically backs up the activity as a single-activity backup, and then restores that.
As you may or may not know, there is a long-standing issue with backup and restore involving shared quetsions, and from the point of view of a single quiz, questions in the course question bank are shared. Well, it is not really in issue in this case. But, when the questions are shared, the duplicate points to the same questions, rather than making new copies.
As April says, one explanation is the for the quiz that duplicated OK, the questions were actually in the quiz-specific part of the question bank. (And you could check that hypthesis though the Moodle UI. You don't have to dive into the database.)