avoid form submisison again in php,mysql

Step 1:

Make a form page say index.php

<html>
<head>
</head>
<body>
	<form name="form1" action="abc.php" method="post">
		First Name :<input type="text" name="firstName"> <br> 
		Second Name :<input type="text" name="secondName"> 
		<input type="submit">
	</form>
    
    <?php
    $_SESSION['token'] = 1; // flag, for not to refresh second page
    ?>
</body>
</html>

Step 2:

Make a php page on which for data is submitting say abc.php

<!DOCTYPE HTML>
<html>
<head>
<title>abcphp</title>
</head>
<body>
<?php
if ($_SESSION['token'] = 1) { // checking flag token for refresh page
                              
    // START accessing form elements
    $firstName = $_POST['firstName'];
    $secondName = $_POST['secondName'];
    // END accessing form elements
    
    // START database connection
    $db_hostname = 'localhost';
    $db_database = 'databseName';
    $db_username = 'userName';
    $db_password = 'password';
    
    $db_server = mysql_connect($db_hostname, $db_username, $db_password);
    mysql_select_db($db_database, $db_server);
    if (! $db_server)
        die("Unable to connect to MySQL: " . mysql_error());
        // END database connection
        
    // START checking record for duplication
    $lastRecord = "SELECT * FROM test234";
    $result = mysql_query($lastRecord);
    
    if (! $result)
        die("Database access failed: " . mysql_error());
    
    $rows = mysql_num_rows($result);
    $identify = 0;
    
    for ($j = 0; $j < $rows; ++ $j) {
        if ($firstName == mysql_result($result, $j, 'name1') &&
                 $secondName == mysql_result($result, $j, 'name2')) {
            $identify = 1; // flag of record duplicates
        }
    }
    // END checking record for duplication
    
    if ($identify == 1) { // checking flag
        echo "record already exists";
        exit();
    } else {
        $query = "INSERT INTO test234(name1,name2) VALUES ('$firstName', '$secondName')";
        $result = mysql_query($query);
        if (! $result)
            die("Database access failed: " . mysql_error());
        
        $_POST['firstName'] = "";
        $_POST['secondName'] = "";
        
        $_SESSION['token'] = 0; // so that use doesn't refresh page
        header('Location: newfile1.php'); // redirection to thank you page
    }
} 

// MySql Query
else {
    header('Location: index.php');
}
// end MySql Query
?>
</body>
</html>

Step 3:

make a thank you page say newfile1.php

<!DOCTYPE HTML>
<html>
<head>
<title>thank you</title>
</head>
<body>
	<?php echo "thank you"; ?>
</body>
</html>