สอบถามหน่อยครับ ว่ามันผิดตรงไหน เกรี่ยวกับ Session PHP

<?php
session_start();
$username = $_POST['username'];
$password = $_POST['password'];

if($username !="admin" && $password !="admin") {
?>

<script type="text/javascript">
window.location = "index.php"
</script>

<?php
exit ;
}
$_SESSION['s_username']= $username;
$_SESSION['s_password']= $password;
?>
<script type="text/javascript">
window.location = "show.php"
</script>

<?php
session_start();

if($_SESSION['s_username']=="") {
?>

<script type="text/javascript">
alert('You are not login \n Please login');
window.location = "index.php"
</script>

<?php
exit ;
}

echo "Welcome , <br>" ;
echo "Your username is : ".$_SESSION['s_username']."<br>";
echo "Your password is : ".$_SESSION['s_password']."<br>";
echo "<a href = 'logout.php'> Logout </a>" ;
?>