7.5 million duplicate questions: how I rescued a Moodle course without wrecking another one's exams

A Moodle course with 7.5 million duplicate questions was breaking the upgrade to the new question bank (mod_qbank) through PostgreSQL's parameter limit, and exhausted 8 GB of PHP memory when I tried to clean it. How I diagnosed it, and how I rescued real exams from another course hiding inside its context.