Nice way let user know email already exists via a flash message

I have made a Registration and Login application with Codeigniter 3.

The Signup form validation makes sure that every email in the users table is unique, as illustrated bellow:



Instead of "The Email field must contain a unique value" below the e-mail field, I want a flash message to let the user know the provided email already exists and that he/she should sigin in instead of signing up:



My signup() function looks like this:

Code:

public function signup()
{
   $this->form_validation->set_rules('first_name', 'First name', 'required');
   $this->form_validation->set_rules('last_name', 'Last name', 'required');
   $this->form_validation->set_rules('email', 'Email', 'required|trim|valid_email|is_unique[users.email]');
   $this->form_validation->set_rules('password', 'Password', 'required|matches[cpassword]');
   $this->form_validation->set_rules('cpassword', 'Confirm password', 'required');
   $this->form_validation->set_error_delimiters('<p class="error">', '</p>');

   if ($this->form_validation->run()) {
       $first_name = $this->input->post('first_name');
       $last_name = $this->input->post('last_name');
       $email =  $this->input->post('email');
       $password = $this->input->post('password');
       $date_created = date('Y-m-d H:i:s');
       $date_updated = date('Y-m-d H:i:s');
       $verification_key = md5($email);
       $active = 0;
       $this->load->model('Usermodel');
       // Create account
       if ($this->Usermodel->user_register($first_name, $last_name, $email, $password, $date_created, $date_updated, $verification_key, $active)) {
           $this->session->set_flashdata("signup_sucess", "Your account was created. Your have to activate it before you can signin. We have send you an activation email at $email.");
       } else {
           $this->session->set_flashdata("signup_failure", "We ware unable to create your account");
       }
       redirect('signup');
   } else {
       $this->load->view('signup');
       if ($email_exists) {
           $this->session->set_flashdata("email_exists", "The email address you provided already exists. Please signup.");
       }
   }
}

I am using an $email_exists function inside an if statement but the function itself does not exist yet.

I want to make use of CI's is_unique inside the $email_exists function. How can I do this?

Is this a good approach to achieving the desired functionality?