stop insert after refresh page

// منع ارسال البيانات بعد عمل تحديث للصفحة

<?php
error_reporting(0);
$localhost="localhost";
$db_user="root";
$db_password="";
$db_name="insert";

$connect_db = mysql_connect($localhost,$db_user,$db_password);
mysql_select_db($db_name);

/*
 CREATE TABLE  `news`.`addnews` (
`id` INT NOT NULL AUTO_INCREMENT PRIMARY KEY ,
`author` VARCHAR( 225 ) NOT NULL ,
`title` VARCHAR( 225 ) NOT NULL ,
`title` LONGTEXT NOT NULL
) ENGINE = MYISAM ;
 */

if(isset($_POST['submit'])){
    $n =$_POST['name'];
    $t =$_POST['title'];
    $c =$_POST['content'];

    if(empty($n) or empty($t) or empty($c)){
        echo'<h3>Please insert all input and click Insert</h3>';
        echo'<meta http-equiv="refresh" content="3;url=insert_2.php"/>';
    }else{
        $q = mysql_query("insert into addnews (author,title,content) values ('$n','$t','$c')")
        or die (" insert not ");
        header('Location: insert_2.php');
        exit;
    }
}

echo "
 <form action='".$SERVER['PHP_SELF']."' method='post'>
    <p> Name : <input type='text' name='name'/> </p>
    <p>Title : <input type='text' name='title'/> </p>
    <p>Content</p> <br/>
    <textarea name='content' rows='8' cols='40'> </textarea>  <br/>
    <p> <input type='submit' name='submit' value='Insert'> </p>
    <input type='hidden' name='add' value='news' />
</form>
";
mysql_close($connect_db);
?>