If you go to the Show non-respondents tab as admin in a feedback module with multiple submissions enabled (and multiple submissions by the same user done) you sill see this error:

Did you remember to make the first column something unique in your call to get_records? Duplicate value '138' found in column 'userid'.

line 1103 of /lib/dml/mysqli_native_moodle_database.php: call to debugging()
line 1380 of /lib/dml/moodle_database.php: call to mysqli_native_moodle_database->get_records_sql()
line 1331 of /lib/dml/moodle_database.php: call to moodle_database->get_records_select()
line 1420 of /lib/dml/moodle_database.php: call to moodle_database->get_records()
line 1003 of /mod/feedback/lib.php: call to moodle_database->get_records_menu()
line 219 of /mod/feedback/show_nonrespondents.php: call to feedback_get_incomplete_users()