For one course I have, for an unknown reason, two grade categories for the same assignment. If the grade_categories table there are two rows with the same course id and the parent category as NULL. This generates an error when upgrading to 1.9.
There is one assignment in the course, which appears twice in the grade_items table, once per each category.
I can't see these anywhere in Moodle, could I just delete the extra ones from the database? If so, should I clean up the *_history tables too?