Help! Can't get Update page to accept changes because "item is already registered"

Hi,

I have created an update page from my insert page similar to chapter 9
in PHP DW8, but I can't get the update to work because it always says
"item is already registered". I have been looking at the code for two
days now and comparing it to the similar author_update.php from the
book, but I just can't find the problem. I'm hoping that someone here
with more knowledge and a fresh set of eyes can spot the problem. Here
is the code:

$editFormAction = $_SERVER['PHP_SELF'];
if (isset($_SERVER['QUERY_STRING'])) {
$editFormAction .= "?" . htmlentities($_SERVER['QUERY_STRING']);
}

$var1_checkModnum = "default";
if (isset($_POST['mod_num'])) {
$var1_checkModnum = $_POST['mod_num'];
}
mysql_select_db($database_belleadmin, $belleadmin);
$query_checkModnum = sprintf("SELECT * FROM products WHERE products.mod_num = %s", GetSQLValueString($var1_checkModnum, "text"));
$checkModnum = mysql_query($query_checkModnum, $belleadmin) or die(mysql_error());
$row_checkModnum = mysql_fetch_assoc($checkModnum);
$totalRows_checkModnum = mysql_num_rows($checkModnum);

mysql_select_db($database_belleadmin, $belleadmin);
$query_divList = "SELECT * FROM divisions ORDER BY div_name ASC";
$divList = mysql_query($query_divList, $belleadmin) or die(mysql_error());
$row_divList = mysql_fetch_assoc($divList);
$totalRows_divList = mysql_num_rows($divList);

mysql_select_db($database_belleadmin, $belleadmin);
$query_groupList = "SELECT * FROM `group` ORDER BY group_name ASC";
$groupList = mysql_query($query_groupList, $belleadmin) or die(mysql_error());
$row_groupList = mysql_fetch_assoc($groupList);
$totalRows_groupList = mysql_num_rows($groupList);

mysql_select_db($database_belleadmin, $belleadmin);
$query_segList = "SELECT * FROM segment ORDER BY seg_name ASC";
$segList = mysql_query($query_segList, $belleadmin) or die(mysql_error());
$row_segList = mysql_fetch_assoc($segList);
$totalRows_segList = mysql_num_rows($segList);

mysql_select_db($database_belleadmin, $belleadmin);
$query_catList = "SELECT * FROM category ORDER BY cat_name ASC";
$catList = mysql_query($query_catList, $belleadmin) or die(mysql_error());
$row_catList = mysql_fetch_assoc($catList);
$totalRows_catList = mysql_num_rows($catList);

$colname_getProduct = "-1";
if (isset($_GET['prod_id'])) {
$colname_getProduct = $_GET['prod_id'];
}
mysql_select_db($database_belleadmin, $belleadmin);
$query_getProduct = sprintf("SELECT * FROM products WHERE prod_id = %s", GetSQLValueString($colname_getProduct, "int"));
$getProduct = mysql_query($query_getProduct, $belleadmin) or die(mysql_error());
$row_getProduct = mysql_fetch_assoc($getProduct);
$totalRows_getProduct = mysql_num_rows($getProduct);


// assume that no match has been found
$alreadyRegistered = false;

// check whether recordset found any matches
if ($totalRows_checkModnum > 0) {
// if found, reset $alreadyRegistered
$alreadyRegistered = true;
}
else {
// go ahead with insert record behavior
if ((isset($_POST["MM_update"])) && ($_POST["MM_update"] == "updateProduct")) {
$updateSQL = sprintf("UPDATE products SET mod_num=%s, div_id=%s, group_id=%s, seg_id=%s, cat_id=%s, title=%s, `desc`=%s, thick=%s, width=%s, height=%s, project=%s, image=%s WHERE prod_id=%s",
GetSQLValueString($_POST['mod_num'], "text"),
GetSQLValueString($_POST['div_id'], "int"),
GetSQLValueString($_POST['group_id'], "int"),
GetSQLValueString($_POST['seg_id'], "int"),
GetSQLValueString($_POST['cat_id'], "int"),
GetSQLValueString($_POST['title'], "text"),
GetSQLValueString($_POST['desc'], "text"),
GetSQLValueString($_POST['thick'], "double"),
GetSQLValueString($_POST['width'], "double"),
GetSQLValueString($_POST['height'], "double"),
GetSQLValueString($_POST['project'], "double"),
GetSQLValueString($_POST['image'], "text"),
GetSQLValueString($_POST['prod_id'], "int"));

mysql_select_db($database_belleadmin, $belleadmin);
$Result1 = mysql_query($updateSQL, $belleadmin) or die(mysql_error());

$updateGoTo = "product_insert.php";
if (isset($_SERVER['QUERY_STRING'])) {
$updateGoTo .= (strpos($updateGoTo, '?')) ? "&" : "?";
$updateGoTo .= $_SERVER['QUERY_STRING'];
}
header(sprintf("Location: %s", $updateGoTo));
}
}

Any help would be greatly appreciated.