With Joseph Rezeau's encouragement, I am suggesting the following simple improvement to quiz editing:
Next to each question in the list of available questions, display a list of quizzes in which that question has been used.

For example

Question-999
Used in Quiz-1, Quiz-2, Quiz-4, Quiz-5

With this list, the teacher can easily spot questions that have been used before (e.g., on practice quizzes) and can avoid accidentally reusing those questions when making up a test. In the above example, I would not choose Question-999 for a test because it has already been used in practice quizzes. Or, I might modify the question and save it as a new question to be used on the test.

Only quizzes in the current course should be listed. The feature can be controlled by an option, since not everyone will be interested in this information.