Thank you for posting the export debug log. You should add the field virtuemart_order_id you are using for group by as export template field. You don't have it added in your template fields so it is not been used in export query. You can add this field and set it unpublished so it is not shown in export file.
If you still have problem with duplicate orders with virtuemart_order_id field in group by then try using virtuemart_order_item_id field as group by field.