Native session in CI 3

Hello guys,
I want to ask , why native session can not be used in CI 3. erornya like this

PHP Code:

A session had already been started - ignoring session_start() 


what should i have to doit if i want use native, my script like this

PHP Code:

<?php 
// print_r($_SESSION);
 $_SESSION['username'] = $this->session->userdata('nama');
$usr = $_SESSION['username']; // Must be already set
session_start();
$_SESSION['username'] = $usr;

// echo $_SESSION['username'];
?>

If you initialize the session library, there should be no reason for you to use session_start() again.

http://www.codeigniter.com/user_guide/li...ssion-data

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