Hello. Why do you repeat $this->CI->load->model('secretaries_model') twice?
"application/libraries/Notifications.php"
/**
* Notifications constructor.
*/
public function __construct()
{
$this->CI =& get_instance();
$this->CI->load->model('providers_model');
$this->CI->load->model('secretaries_model');
$this->CI->load->model('secretaries_model');
$this->CI->load->model('admins_model');
$this->CI->load->model('appointments_model');
$this->CI->load->model('settings_model');
$this->CI->load->library('ics_file');
$this->CI->load->library('timezones');
$this->CI->config->load('email');
}