help modding an old script

[Christo]

I wrote this last year, when i was getting into PHP, it was easy at the time
to write, however I have since not held my intrest in PHP so am looking for
some help adding to what is already here

here is the code

//CODE

<?php

$error = "http://www.yoursite.com/shoutb0x11/error.php";
$filename = "data.dat";
$box = "http://www.yoursite.com/shoutb0x11/shout.php";
$max = "2048";
$message = stripslashes($message);

if(filesize($filename) > $max) {
unlink($filename);
}
if (empty($name) || empty($message)) { header( "Location: $error" );
} else {
header( "Location: $box");
$a = fopen( $filename, "a") or die ("could not open $filename for writing
please check file permissions");
flock($a, LOCK_SH);
fwrite($a, "<font face=verdana size=1><b>$name</b> : $message</font>
<hr noshade size=1 color=000000>");
flock($a, LOCK_UN);
fclose($a);
}
?>

//CODE END

I want to be able to stop people from writing to the data.dat file
constantly over and over, or flooding the file, or maybe even prevent
identicle entries, however just a quick and easy way to implement to stop
posts being sent one after the other over and over would help greatly.

If anyone could maybe provide a snippet of code and instructions on where to
place the code i would greatly appreciate it, or if anyone could point me in
the direction of an example or information about adding a feature like this
i would much appreciate it

Thanks In Advance

Christo