IPN listener always has "Completed" status

[cockrachecommx]

Here is the problem, I have a listener that goes as follows: 

 $raw_post_data = file_get_contents('php://input');
    $raw_post_array = explode('&',$raw_post_data);
    $myPost = array();
    foreach($raw_post_array as $keyval){
        $keyval = explode("=",$keyval);
        if(count($keyval) == 2)
            $myPost[$keyval[0]] = urldecode($keyval[1]);
    }
    $req = 'cmd=_notify-validate';
    if(function_exists('get_magic_quotes_gpc')){
        $get_magic_quotes_exists = true;
    }
    foreach($myPost as $key => $value){
       if($get_magic_quotes_exists == true && get_magic_quotes_gpc() == 1) {
            $value = urlencode(stripslashes($value));
        } else {
            $value = urlencode($value);
        }
        $req .= "&$key=$value";
    }
    
    $ch = curl_init('https://www.paypal.com/cgi-bin/webscr');  
        curl_setopt($ch, CURLOPT_HTTP_VERSION, CURL_HTTP_VERSION_1_1);
        curl_setopt($ch, CURLOPT_POST, 1);
        curl_setopt($ch, CURLOPT_RETURNTRANSFER,1);
        curl_setopt($ch, CURLOPT_POSTFIELDS, $req);
        curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, 1);
        curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 2);
        curl_setopt($ch, CURLOPT_FORBID_REUSE, 1);
        curl_setopt($ch, CURLOPT_HTTPHEADER, array('Connection: Close'));

        if( !($res = curl_exec($ch)) ) {
            curl_close($ch);
            exit;
        }
        curl_close($ch);

        if (strcmp ($res, "VERIFIED") == 0) {
                     
            if($_POST["payment_status"]==="Completed"){
                  /// here I have some code to do an insert in my db
            }
                   [... continues ...]

The problem is that my IPN is inserting like 5 or more regirsters in my data base, every time I do a test payment with a dollar...

I was given the advice to check if $_POST["payment_status"] was "Completed" to do my insert, and also check $_POST["txn_id"] to check if it was unique.

I haven't done the last option, I see that the 5 problematic inserts have the same txn_id, so I'm pretty sure the "payment_status" value is being "Completed" every time.

So... How could I prevent my code to insert multiple transactions with the same "txn_id"... and instead insert only ONE, but allowing me to know if the transaction was valid or not.