Probleme A session had already been started

<?php 
session_start();
$cnx = mysql_connect( "localhost", "root", "" ) ;
$db = mysql_select_db( "image" ) ;
// on teste si nos variables sont définies
	if (isset($_POST['email']) and isset($_POST['password'])) {
	$email=$_POST['email'];
	$password=$_POST['password'];
	$query = "SELECT * FROM `users` WHERE email='$email' and password='$password'";
	$res = mysql_query($query) or die(mysql_error());
	$count= mysql_num_rows($res);
	if ($count == 1){
		session_start ();
		$_SESSION['email'] = $email;
		header ('location: useer.php');
		}
	else {
		$query = "SELECT * FROM `admin` WHERE email='$email' and password='$password'";
		$res = mysql_query($query) or die(mysql_error());
		$count= mysql_num_rows($res);
			if ($count == 1){
				session_start ();
				$_SESSION['email'] = $email;
						header ('location: administrateur.php');
				}
	}
	}

?>