This seems to be caused by the amount of questions (and question banks!) attached to the quiz.
I use a non-tech way to solve the issue, as I don't have access to our database.
Open your existing quiz, and use the "Export Quiz Block" plugin to export the quiz as a Wordtable file. Open that file on your computer, and save it as a Word document (docx).
Still in the quiz you have issues with (you must be in the quiz), select Question Bank, Import. Under the General selection, select the category named "Default for (QuizName)", where QuizName is the name of the quiz you have issues with - it will be at the very top of the selectable categories list. Also, ensure you DESELECT both "Get context, and Get Category from file".
You will now have a category which is only visible when editing THAT quiz, and contains only the questions for THAT quiz.
Next, in that particular quiz, using "Edit Quiz", select ALL the questions and delete them. Now save.
Now, go back to editing that quiz but, this time, when adding questions, select "from question bank" and make sure you select the very top "Default for (Quizname)" category you created earlier. Select ALL the questions and add them and save.
Now, when you try to duplicate the quiz, it will no longer try to bring every single question bank with it, and should duplicate just fine. An alternative is to do the duplication when the "problem" quiz is empty, and then, in both the "problem" quiz and its duplicate, you can import from the "Default for (Quizname)" category to repopulate with the questions.
It was a mouthful, but it's quite fast and quite easy, and appears to solve the immediate issue of being unable to duplicate quizzes, if the problem is caused by having used higher level (Course or System) question banks which contain many thousands of questions.