Restoring as merge fails due to gradebook

Debug info: Duplicate entry '3-1' for key 'mdl_gradgrad_useite_uix'

INSERT INTO mdl_grade_grades (userid,rawgrade,rawgrademax,rawgrademin,rawscaleid,usermodified,finalgrade,hidden,locked,locktime,exported,overridden,excluded,feedback,feedbackformat,information,informationformat,timecreated,timemodified,itemid) VALUES(?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?,?)

[array (

0 => '3',

1 => NULL,

2 => '100.00000',

3 => '0.00000',

4 => NULL,

5 => NULL,

6 => '1.00000',

7 => '0',

8 => '0',

9 => '0',

10 => '0',

11 => '0',

12 => '0',

13 => NULL,

14 => '0',

15 => NULL,

16 => '0',

17 => NULL,

18 => NULL,

19 => '1',

)]

Error code: dmlwriteexception

Stack trace:

line 451 of /lib/dml/moodle_database.php: dml_write_exception thrown

line 1079 of /lib/dml/mysqli_native_moodle_database.php: call to moodle_database->query_end()

line 1121 of /lib/dml/mysqli_native_moodle_database.php: call to mysqli_native_moodle_database->insert_record_raw()

line 237 of /backup/moodle2/restore_stepslib.php: call to mysqli_native_moodle_database->insert_record()

line 137 of /backup/util/plan/restore_structure_step.class.php: call to restore_gradebook_structure_step->process_grade_grade()

line 103 of /backup/util/helper/restore_structure_parser_processor.class.php: call to restore_structure_step->process()

line 151 of /backup/util/xml/parser/processors/grouped_parser_processor.class.php: call to restore_structure_parser_processor->dispatch_chunk()

line 91 of /backup/util/helper/restore_structure_parser_processor.class.php: call to grouped_parser_processor->postprocess_chunk()

line 148 of /backup/util/xml/parser/processors/simplified_parser_processor.class.php: call to restore_structure_parser_processor->postprocess_chunk()

line 92 of /backup/util/xml/parser/processors/progressive_parser_processor.class.php: call to simplified_parser_processor->process_chunk()

line 186 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser_processor->receive_chunk()

line 274 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser->publish()

line ? of unknownfile: call to progressive_parser->end_tag()

line 175 of /backup/util/xml/parser/progressive_parser.class.php: call to xml_parse()

line 154 of /backup/util/xml/parser/progressive_parser.class.php: call to progressive_parser->parse()

line 110 of /backup/util/plan/restore_structure_step.class.php: call to progressive_parser->process()

line 181 of /backup/util/plan/base_task.class.php: call to restore_structure_step->execute()

line 177 of /backup/util/plan/base_plan.class.php: call to base_task->execute()

line 167 of /backup/util/plan/restore_plan.class.php: call to base_plan->execute()

line 343 of /backup/controller/restore_controller.class.php: call to restore_plan->execute()

line 184 of /backup/util/ui/restore_ui.class.php: call to restore_controller->execute_plan()

line 77 of /backup/restore.php: call to restore_ui->execute()