warning message in adding data that already input using codeigniter

function enrollstudent(){
        if($this->session->userdata('logged_in'))
        {
            
     $this->output->set_header('Expires: Sat, 26 Jul 1997 05:00:00 GMT');
     $this->output->set_header('Cache-Control: no-cache, no-store, must-revalidate, max-age=0');
     $this->output->set_header('Cache-Control: post-check=0, pre-check=0', FALSE);
     $this->output->set_header('Pragma: no-cache');
               $session_data = $this->session->userdata('logged_in');
           
            $data['Username'] = $session_data['Username'];
            $this->load->view('StudentEnroll', $data);
        } else{
            redirect('welcome', 'refresh');
        }
        if((!isset($session_data) || $session_data !=TRUE)) {
          
            redirect('welcome', 'refresh');
        }
    }
  

  function addstatusofenroll($id){
        
           $this->form_validation->set_rules('status', 'Status', 'callback_status_check');
 if($this->form_validation->run() == FALSE) 
        {
        $this->enrollstudent();}
        else{
              $data= array(
           'Id'=> $id ,
            'Statusofenrollment' => $this->input->post('status')
             
            
            
            );
              $this->db->insert('statusofenrollment',$data);
          
             $this->session->set_flashdata('category_success', 'Successfully Add!');
               $this->load->view('StudentEnroll');
                         }
                             
                         
    }
    
  

   public function callback_status_check($roll) 
    {
       $this->db->where('Id', $roll);
    $query = $this->db->get('statusofenrollment');
    $count_row = $query->num_rows();
    if ($count_row > 0) 
    {
        echo 'Data Already exist';
       $this->session->set_flashdata('warning', 'Data already exists');

              return FALSE;
    } 
    else 
    {
        return TRUE;
    }

       
    }