php if orderID exists give new order number

I am having a trouble with a shopping cart i am setting up. basically if the order is processed sucessfully it returns a value to the database of PAID and next time the order is placed a new orderID is issues, however when the order fails it return the result FAILED, what is happening is when you try to re order it is issuing the same orderID as the failed orderID do when the user try to proceed the the gateway i am getting a DUPLICATE ID error. on the failed-order.pho i have killed the orderID (session_unregister("OrderID");)

this is what i thought i would need to do so when the user is sent to failed-order.php this would kill the session then give the user a new orderID?

I have included the script on the checkout page ( only the parts relating to the orderID)

// *** Retrieve X ID ***

if (!session_id()) session_start();

$XC_OrderIdSessionVar = "OrderID";

if (!isset($_SESSION[$XC_OrderIdSessionVar])) {

  // Get a unique OrderID number and save to session.

  $XC_tableName = "LOTTIE_nextorder";

  $XC_fieldName = "NextOrderID";

  mysql_select_db($database_lotties, $lotties);

  $XC_IdSource = "select " . $XC_fieldName . " from " .  $XC_tableName;

  $XC_rsId = mysql_query($XC_IdSource, $lotties) or die(mysql_error());

  $row_XC_rsId = mysql_fetch_assoc($XC_rsId);

  $_SESSION[$XC_OrderIdSessionVar] = $row_XC_rsId[$XC_fieldName];

  $$XC_OrderIdSessionVar = $_SESSION[$XC_OrderIdSessionVar];

  session_register($XC_OrderIdSessionVar);

  $XC_next = $_SESSION[$XC_OrderIdSessionVar] + 1;

  $XC_upd = "update " . $XC_tableName . " set " . $XC_fieldName . " = " . $XC_next;

  $XC_rsId = mysql_query($XC_upd, $lotties) or die(mysql_error());

  $XC_rsId = null;

}

if ((isset($_POST["MM_insert"])) && ($_POST["MM_insert"] == "form1")) {

  $insertSQL = sprintf("INSERT INTO LOTTIE_orders (OrderID, CustomerID, OrderDate, Shipping, Discount, Tax, Total) VALUES (%s, %s, %s, %s, %s, %s, %s)",

                       GetSQLValueString($_POST['OrderID'], "text"),

                       GetSQLValueString($_POST['CustomerID'], "int"),

                       GetSQLValueString($_POST['OrderDate'], "date"),

                       GetSQLValueString($_POST['Shipping'], "double"),

                       GetSQLValueString($_POST['Discount'], "double"),

                       GetSQLValueString($_POST['Tax'], "double"),

                                                     GetSQLValueString($_POST['XC_Amount'], "double"));

// *** Save XCart contents to table ***

require_once('XCInc/XCsaveAction.inc');

if (isset($_GET['XC_SaveCartToTable']) && ($_GET['XC_SaveCartToTable'] == "1")) {

  $XC_destColName = array("ProductID","Quantity","","","Total");

  $XC_destColType = array("str","str","str","num","num");

  $XC_orderId = $_SESSION['OrderID'];

  $XC_tableName = "LOTTIE_orderdetails";

  $XC_OrderIDCol = "OrderID";

  $XC_OrderIDType = "num";

  $XC_AddToTableRedirect = "../HostedSample/Process.php?$x_reqstr";

  $XC_conName = "lotties";

  require_once('XCInc/SaveXCartToTable.inc');

}

  mysql_select_db($database_lotties, $lotties);

  $Result1 = mysql_query($insertSQL, $lotties) or die(mysql_error());

what do i need to do to rectify this issue

thanks in advance