Gradebook

Hi there. We have used Moodle for many years, teaching around a thousand students at a time in a dozen or so courses. We’re currently running Moodle 3.11.5+ (Build: 20220208). Twice in recent weeks we have encountered a worrying situation in which student submissions have stopped working for a few minutes, before resuming with no intervention on our part. The errors in the Apache logs look like this:

Default exception handler: Error writing to database Debug: 
Duplicate entry '154-844' for key 'mdl_grade_grades.mdl_gradgrad_useite_uix'
INSERT INTO mdl_grade_grades (itemid,userid,rawgrade,rawgrademax,rawgrademin,
rawscaleid,usermodified,finalgrade,hidden,locked,locktime,exported,
overridden,excluded,timecreated,timemodified,aggregationstatus,
aggregationweight) 
VALUES(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)
[array (
    0 => 844,
    1 => '154',
    2 => NULL,
    3 => '100.00000',
    4 => '0.00000',
    5 => NULL,
    6 => NULL,
    7 => NULL,
    8 => 0,
    9 => 0,
    10 => 0,
    11 => 0,
    12 => 0,
    13 => 0,
    14 => NULL,
    15 => NULL,
    16 => 'unknown',
    17 => NULL,
)]
Error code: dmlwriteexception
* line 489 of /lib/dml/moodle_database.php: dml_write_exception thrown
* line 1357 of /lib/dml/mysqli_native_moodle_database.php: call to
moodle_database->query_end()
* line 1403 of /lib/dml/mysqli_native_moodle_database.php: call to
mysqli_native_moodle_database->insert_record_raw()
* line 349 of /lib/grade/grade_object.php: call to
mysqli_native_moodle_database->insert_record()
* line 1033 of /lib/grade/grade_grade.php: call to
grade_object->insert()
* line 653 of /lib/grade/grade_category.php: call to grade_grade->insert()
* line 541 of /lib/grade/grade_category.php: call to
grade_category->aggregate_grades()
* line 809 of /lib/grade/grade_item.php: call to
grade_category->generate_grades()
* line 1276 of /lib/gradelib.php: call to grade_item->regrade_final_grades()
* line 448 of /lib/gradelib.php: call to grade_regrade_final_grades()
* line 812 of /mod/quiz/lib.php: call to grade_get_grades()
* line 1212 of /mod/quiz/lib.php: call to quiz_grade_item_update()
* line 139 of /mod/quiz/lib.php: call to quiz_after_add_or_update()
* line 612 of /course/modlib.php: call to quiz_update_instance()
* line 166 of /course/modedit.php: call to update_moduleinfo()

There were about 200 of these messages during the first event, and about 20 in the second. 

Any suggestions as to what might be going on and how to diagnose/fix it would be greatly appreciated.

A similar problem was reported in 2017 here: https://moodle.org/mod/forum/discuss.php?d=348924 although this was ocurring during course creation, not during quiz submission.

Thanks, Joff.