Noticed whilst confirming the fix for MDL-31589:

As an administrator:

• Create a new graded item. I confirmed with both an Online Text assignment, and Lesson
  As a student
• Submit an attempt at said graded item such that a gradebook entry is now present in mdl_grade_grades

As administrator:

• Backup the course:
  • Include User Data
  • Do not include the 'General' section, but do include the section with the activity in question

• Restore the backup:
  • Go through all steps, merging in to the existing course

What you see:
Bang from duplicate userid, itemid index on grade_grades

It looks like backup/moodle2/restore_stepslib.php restore_gradebook_structure_step::process_grade_grade always tries an insert, even though process_grade_item may update an existing record

Not sure whether we should update the existing grade_grade record or ignore the newest data?