Is it possible to add the idnumber also to the method 
duplicate_course in externallib.
 
Passing the param idnumber to the method core_course_duplicate_course should also be possible.
 
The two methods for this could be:
 
public static function duplicate_course_parameters()

{ return new external_function_parameters( array( 'courseid' => new external_value(PARAM_INT, 'course to duplicate id'), 'fullname' => new external_value(PARAM_TEXT, 'duplicated course full name'), 'shortname' => new external_value(PARAM_TEXT, 'duplicated course short name'), 'categoryid' => new external_value(PARAM_INT, 'duplicated course category parent'), 'visible' => new external_value(PARAM_INT, 'duplicated course visible, default to yes', VALUE_DEFAULT, 1), 'options' => new external_multiple_structure( new external_single_structure( array( 'name' => new external_value(PARAM_ALPHAEXT, 'The backup option name: "activities" (int) Include course activites (default to 1 that is equal to yes), "blocks" (int) Include course blocks (default to 1 that is equal to yes), "filters" (int) Include course filters (default to 1 that is equal to yes), "users" (int) Include users (default to 0 that is equal to no), "enrolments" (int) Include enrolment methods (default to 1 - restore only with users), "role_assignments" (int) Include role assignments (default to 0 that is equal to no), "comments" (int) Include user comments (default to 0 that is equal to no), "userscompletion" (int) Include user course completion information (default to 0 that is equal to no), "logs" (int) Include course logs (default to 0 that is equal to no), "grade_histories" (int) Include histories (default to 0 that is equal to no)' ), 'value' => new external_value(PARAM_RAW, 'the value for the option 1 (yes) or 0 (no)' ) ) ), 'Course duplication options', VALUE_DEFAULT, array() ), *'idnumber' => new external_value(PARAM_TEXT, 'idnumber for the course', VALUE_OPTIONAL),* ) ); }

/**

• Duplicate a course
  *
• @param int $courseid
• @param string $fullname Duplicated course fullname
• @param string $shortname Duplicated course shortname
• @param int $categoryid Duplicated course parent category id
• @param int $visible Duplicated course availability
• @param array $options List of backup options
• @return array New course info
• @since Moodle 2.3
  */
  public static function duplicate_course($courseid, $fullname, $shortname, $categoryid, $visible = 1, $options = array(), $idnumber = null) {
  global $CFG, $USER, $DB;
  require_once($CFG->dirroot . '/backup/util/includes/backup_includes.php');
  require_once($CFG->dirroot . '/backup/util/includes/restore_includes.php');

// Parameter validation.
$params = self::validate_parameters(
self::duplicate_course_parameters(),
array(
'courseid' => $courseid,
'fullname' => $fullname,
'shortname' => $shortname,
'categoryid' => $categoryid,
'visible' => $visible,
'options' => $options,
'idnumber' => $idnumber
)
);

// Context validation.

if (! ($course = $DB->get_record('course', array('id'=>$params['courseid']))))

{ throw new moodle_exception('invalidcourseid', 'error'); }

// Category where duplicated course is going to be created.
$categorycontext = context_coursecat::instance($params['categoryid']);
self::validate_context($categorycontext);

// Course to be duplicated.
$coursecontext = context_course::instance($course->id);
self::validate_context($coursecontext);

$backupdefaults = array(
'activities' => 1,
'blocks' => 1,
'filters' => 1,
'users' => 0,
'enrolments' => backup::ENROL_WITHUSERS,
'role_assignments' => 0,
'comments' => 0,
'userscompletion' => 0,
'logs' => 0,
'grade_histories' => 0
);

$backupsettings = array();
// Check for backup and restore options.
if (!empty($params['options'])) {
foreach ($params['options'] as $option) {

// Strict check for a correct value (allways 1 or 0, true or false).
$value = clean_param($option['value'], PARAM_INT);

if ($value !== 0 and $value !== 1)

{ throw new moodle_exception('invalidextparam', 'webservice', '', $option['name']); }

if (!isset($backupdefaults[$option['name']])) { throw new moodle_exception('invalidextparam', 'webservice', '', $option['name']); }

$backupsettings[$option['name']] = $value;
}
}

// Capability checking.

// The backup controller check for this currently, this may be redundant.
require_capability('moodle/course:create', $categorycontext);
require_capability('moodle/restore:restorecourse', $categorycontext);
require_capability('moodle/backup:backupcourse', $coursecontext);

if (!empty($backupsettings['users']))

{ require_capability('moodle/backup:userinfo', $coursecontext); require_capability('moodle/restore:userinfo', $categorycontext); }

// Check if the shortname is used.
if ($foundcourses = $DB->get_records('course', array('shortname'=>$shortname))) {
foreach ($foundcourses as $foundcourse)

{ $foundcoursenames[] = $foundcourse->fullname; }

$foundcoursenamestring = implode(',', $foundcoursenames);
throw new moodle_exception('shortnametaken', '', '', $foundcoursenamestring);
}

// Backup the course.

$bc = new backup_controller(backup::TYPE_1COURSE, $course->id, backup::FORMAT_MOODLE,
backup::INTERACTIVE_NO, backup::MODE_SAMESITE, $USER->id);

foreach ($backupsettings as $name => $value) {
if ($setting = $bc->get_plan()->get_setting($name))

{ $bc->get_plan()->get_setting($name)->set_value($value); }

}

$backupid = $bc->get_backupid();
$backupbasepath = $bc->get_plan()->get_basepath();

$bc->execute_plan();
$results = $bc->get_results();
$file = $results['backup_destination'];

$bc->destroy();

// Restore the backup immediately.

// Check if we need to unzip the file because the backup temp dir does not contains backup files.
if (!file_exists($backupbasepath . "/moodle_backup.xml"))

{ $file->extract_to_pathname(get_file_packer('application/vnd.moodle.backup'), $backupbasepath); }

// Create new course.
$newcourseid = restore_dbops::create_new_course($params['fullname'], $params['shortname'], $params['categoryid']);

$rc = new restore_controller($backupid, $newcourseid,
backup::INTERACTIVE_NO, backup::MODE_SAMESITE, $USER->id, backup::TARGET_NEW_COURSE);

foreach ($backupsettings as $name => $value) {
$setting = $rc->get_plan()->get_setting($name);
if ($setting->get_status() == backup_setting::NOT_LOCKED)

{ $setting->set_value($value); }

}

if (!$rc->execute_precheck()) {
$precheckresults = $rc->get_precheck_results();
if (is_array($precheckresults) && !empty($precheckresults['errors'])) {
if (empty($CFG->keeptempdirectoriesonbackup))

{ fulldelete($backupbasepath); }

$errorinfo = '';

foreach ($precheckresults['errors'] as $error) { $errorinfo .= $error; }

if (array_key_exists('warnings', $precheckresults)) {
foreach ($precheckresults['warnings'] as $warning) { $errorinfo .= $warning; }
}

throw new moodle_exception('backupprecheckerrors', 'webservice', '', $errorinfo);
}
}

$rc->execute_plan();
$rc->destroy();

$course = $DB->get_record('course', array('id' => $newcourseid), '*', MUST_EXIST);
$course->fullname = $params['fullname'];
$course->shortname = $params['shortname'];
$course->visible = $params['visible'];
$course->idnumber = $params['idnumber'];

// Set shortname and fullname back.
$DB->update_record('course', $course);

if (empty($CFG->keeptempdirectoriesonbackup)) { fulldelete($backupbasepath); }

// Delete the course backup file created by this WebService. Originally located in the course backups area.
$file->delete();

return array('id' => $course->id, 'shortname' => $course->shortname);
}