数组每行输入两次

我有一个模型，该模型具有查询学生注册的科目的功能。

public function getAllSubjects($username){
    $this->db->select('student.username, subjects.subject_name, teacher.firstname as TeachersFirstName, teacher.surname as TeacherSurname');
    $this->db->from('users student');
    $this->db->join('student_subjects', 'student.username = student_subjects.student_username', 'inner');
    $this->db->join('subjects', 'student_subjects.subject_id = subjects.subject_id', 'inner');
    $this->db->join('users teacher', 'subjects.teacher_username = teacher.username', 'inner');
    $this->db->where('student.username', $username);
    $studentSubjQuery = $this->db->get('users');
    return $studentSubjQuery->result_array();
}

在控制器中，我从模型中调用函数，并将返回的数组存储到数据中，并将其传递给视图。现在，当我尝试打印数组时，行在数组中存储两次。下面是打印出来的数组:

    Array
(
    [0] => Array
        (
            [username] => dirkjan021
            [subject_name] => Chemistry
            [TeachersFirstName] => Teacher 1
            [TeacherSurname] => Teacher 1 Surname
        )
    [1] => Array
        (
            [username] => dirkjan021
            [subject_name] => Chemistry
            [TeachersFirstName] => Teacher 1
            [TeacherSurname] => Teacher 1 Surname
        )
    [2] => Array
        (
            [username] => dirkjan021
            [subject_name] => Physics
            [TeachersFirstName] => Teacher 1
            [TeacherSurname] => Teacher 1 Surname
        )
    [3] => Array
        (
            [username] => dirkjan021
            [subject_name] => Physics
            [TeachersFirstName] => Teacher 1
            [TeacherSurname] => Teacher 1 Surname
        )
)

控制器代码:

public function acads()
{
    if (!$this->session->userdata('username')) {
        // Redirect to homepage
        redirect('../');
    } else {
        // Allow Access to User's Acads
        $data['userInfo'] = $this->session->all_userdata();
        // Query User Subjects
        $this->load->model('usersubject_model');
        $data['userSubjects'] = $this->usersubject_model->getAllSubjects($this->session->userdata('username'));
        $this->load->view('view_userAcads', $data);
    }
}

更新:下面是根据'$this->output->profiler(TRUE)'的查询:

SELECT `student`.`username`, `subjects`.`subject_name`, `teacher`.`firstname` as TeachersFirstName, `teacher`.`surname` as TeacherSurname
FROM (`users` student, `users`)
INNER JOIN `student_subjects` ON `student`.`username` = `student_subjects`.`student_username`
INNER JOIN `subjects` ON `student_subjects`.`subject_id` = `subjects`.`subject_id`
INNER JOIN `users` teacher ON `subjects`.`teacher_username` = `teacher`.`username`
WHERE `student`.`username` =  'dirkjan021'