Alert double input menggunakan jquery

Bagaimana caranya agar Item yang di masukan tidak boleh sama ? Harusnya bila item yg di add sama muncul alert 'Item already input'. Alert pada saat klik save dan otomatis data tidak tersimpan atau update.

CODING JQUERY ADD ITEM

function addMaterialItem()
{
	var vHTML = '';

	vHTML = '<div class="materialItem add">';
	i += 1;
	vHTML += '<div>';

vHTML += '<div style="float:left; width:195px;"><input type="text" id="item_' + i + '" name="item_' + i + '" title="Item" class="formElement items" style="width:190px;"  value="" /><input type="text" id="items_id_' + i + '" name="items_id_' + i + '" class="formElement items_id" value="" /><input type="text" class="formElement rows" id="rows_' + i + '" name="rows[]" value="' + i + '" /></div>';

vHTML += '<div style="float:left; margin-left:2px; width:40px;"><input type="text" id="item_satuan_' + i + '" name="item_satuan_' + i + '" title="Satuan" class="formElement item_satuan" style="width:35px;" onFocus="$(this).removeClass(\'missingInfo\')" value="" readonly /></div>';

vHTML += '<div style="float:left; margin-left:2px; width:50px;"><input type="text" id="qty_' + i + '" name="qty_' + i + '" title="Quantity" class="formElement qty" style="width:45px; text-align:center;" value="" onChange="CekStock(this);" /><input type="hidden" id="stok_' + i + '" name="stok_' + i + '" class="formElement stok" value=""/></div>';

vHTML += '<div style="float:left; margin-left:2px; width:400px;"><input type="text" id="remark_' + i + '" name="remark_' + i + '" title="Remark" class="formElement remark" style="width:395px; text-align:center;" onChange="sumTotal(this); updatePETotals();" value=""/><input type="hidden" id="total_wo_tax_' + i + '" name="total_wo_tax_' + i + '" title="Total WO/ Tax" class="formElement total_wo_tax" value=""  /><input type="hidden" id="total_' + i + '" name="total_' + i + '" title="Total W/ Tax" class="formElement total"  value=""  /></div>';

vHTML += '<div style="float:right; margin-top:2px; margin-right:1px;"><img src="../../images/delete_12.png" style="cursor:pointer;" width="12" height="12" border="0" title="Remove this item" onClick="removeMaterialItem(this);" /></div>';

vHTML += '<div style="clear:both;"></div>';
vHTML += '</div>';
vHTML += '</div>';

	vObj = $(vHTML);

	$('#materialList').append(vObj).scrollTop($('#materialList').scrollTop()+30);



	//Autocompletes

	itemAutocomplete(vObj);
	updatePETotals();
	return true;
}

CODING FILTER SAVE JQUERY (YANG MAU DI EDIT)

function preSubmit()
{
	if (!confirm('Do you really want to continue?'))
		return false;

	if ($('.items_id').val() != $('.items_id').val())
		{
		alert('Item already input.');
		return false;
		}

	if(!$('.qty').val())
		{
		alert('- Quantity field is mandatory.');
		return false;
		}

		//Disable buttons
	$('input.button').prop('disabled',true);

}

CODING INSERT/UPDATE

<?php
session_start();
//koneksi database
 include "../../inc/inc.koneksi.php";
 include "../../inc/fungsi_tanggal.php";

 $Code 			= $_POST['code'];
 $description 	= $_POST['description'];
 $RequestBy 	= $_POST['request_by_id'];
 $Request2By 	= $_POST['request2_by_id'];
 $Date			= jin_date_sql($_POST['date']);



 $users		= $_SESSION[namalengkap];


$text	= "SELECT max(Code) as noakhir FROM outbound_code";
$sql 	= mysql_query($text);
$row	= mysql_num_rows($sql);
if ($row>0){
	$r=mysql_fetch_array($sql);
	$noakhir = $r[noakhir];
	$no	= (int) substr($noakhir,0,7);
	$no++;
	$kode	= sprintf("%07s",$no);
	$data['kode']	= $kode;
}else{
	$data['kode']	= '0000001';
}

if(!empty($RequestBy)){
	$Request2By=$RequestBy;
	}else{
	$RequestBy=$Request2By;}

$tgl	= date("y");
$VoucherNo="OUT-".$tgl."1-".$kode;

// ---------------------Update baru------------------

//Input Staff
 $sql = mysql_query("SELECT Code
				   FROM outbound
				   WHERE Code='$Code'");

 $row	= mysql_num_rows($sql);

 if ($row>0){

	 $input	= "UPDATE outbound SET
					outbound_date = '$Date',
					RequestBy	= '$RequestBy'
					WHERE Code='$Code'";
	mysql_query($input);


foreach ($_POST['rows'] as $key => $count ){
		 $RowID 		= $_POST['rows_id_'.$count];
		 $Items 		= $_POST['items_id_'.$count];
		 $Satuan 		= $_POST['item_satuan_'.$count];
		 $Qty 			= $_POST['qty_'.$count];
		 $Price 		= $_POST['price_'.$count];
		 $Remark 		= $_POST['remark_'.$count];
		 $Total			= $_POST['total_'.$count];
		 $TotalWOTax	= $_POST['total_wo_tax_'.$count];

	$sqlD = mysql_query("SELECT ID
				   FROM outbound_detail
				   WHERE ID='$RowID'");
	$rowD	= mysql_num_rows($sqlD);

$sqlE = mysql_query("SELECT qty
				   FROM outbound_detail
				   WHERE ID='$RowID'");
$rowE	= mysql_fetch_array($sqlE);

	if ($rowD>0){
		$input	= "UPDATE outbound_detail SET
					ID_Product	= '$Items',
					Satuan		= '$Satuan',
					qty 		= '$Qty',
					remark 		= '$Remark'

					WHERE ID='$RowID'";
	mysql_query($input);

	$queryS	= "UPDATE stok_barang SET
					qty			= qty+'$rowE[qty]'-'$Qty'
					WHERE ID_Product='$Items'";
	mysql_query($queryS);

	}else{
		if(!empty($Items)){
		$queryH = "INSERT INTO outbound_detail
			  (Code, ID_Product, Satuan, qty, remark)
		   VALUES
			  ('$Code', '$Items', '$Satuan', '$Qty', '$Remark')";
		mysql_query($queryH);
		}

		$queryS	= "UPDATE stok_barang SET
					qty			= qty-'$Qty'
					WHERE ID_Product='$Items'";
	mysql_query($queryS);
	}
	}

$sqlH = mysql_query("SELECT ID
				   FROM outbound_history
				   WHERE Code='$Code' AND Activity='Edited'");
$rowH	= mysql_num_rows($sqlH);

if ($rowH>0){
		$input	= "UPDATE outbound_history SET
					LoggedBy 		= '$users',
					Date			= Now()

					WHERE  Code='$Code' AND Activity='Edited'";
	mysql_query($input);
}else{
	$queryH = "INSERT INTO outbound_history
			  (Code, LoggedBy, Activity, Date)
		   VALUES
			  ('$Code', '$users', 'Edited', Now())";
		mysql_query($queryH);
}

	echo "<script language='javascript' type='text/javascript'>
				window.parent.location.reload();
		 </script>";

}else{


// --------------------Insert baru-------------------

$query = "INSERT INTO outbound
			  (Code, outbound_date, status, pic, RequestBy, remark)
		   VALUES
			  ('$VoucherNo', NOW(), 'New', '$users', '$RequestBy','$remark')";
	mysql_query($query);


	$queryC = "INSERT INTO outbound_code (Code)
		   VALUES
		  ('$kode')";
	mysql_query($queryC);

foreach ($_POST['rows'] as $key => $count ){
         $Items 		= $_POST['items_id_'.$count];
		 $Satuan 		= $_POST['item_satuan_'.$count];
		 $Qty 			= $_POST['qty_'.$count];
		 $remark 		= $_POST['remark_'.$count];
		 $Total			= $_POST['total_'.$count];
		 $TotalWOTax	= $_POST['total_wo_tax_'.$count];

/*$sqlC = mysql_query("SELECT qty as qty
				FROM stok_barang
					WHERE nama_barang='$Items'");
$rowC	= mysql_fetch_array($sqlC);
*/

if(!empty($Items)){

	$queryH = "INSERT INTO outbound_detail
			  (Code, ID_Product, Satuan, qty, remark)
		   VALUES
			  ('$VoucherNo', '$Items', '$Satuan', '$Qty', '$remark')";
	mysql_query($queryH);

	$queryS	= "UPDATE stok_barang SET
					qty			= qty-'$Qty'
					WHERE ID_Product='$Items'";
	mysql_query($queryS);
}
}

$sqlH = mysql_query("SELECT ID
				   FROM outbound_history
				   WHERE Code='$Code' AND Activity='Created'");
$rowH	= mysql_num_rows($sqlH);
	if ($rowH>0){
		$input	= "UPDATE outbound_history SET
					LoggedBy 			= '$users',
					LoggedDate 			= Now()

					WHERE Code='$Code' AND Activity='Created'";
		 mysql_query($input);
	}else{
		$queryH = "INSERT INTO outbound_history (Code, LoggedBy, Activity, Date)
				VALUES
					('$VoucherNo', '$users', 'Created', Now())";
		mysql_query($queryH);
	}

echo "<script language='javascript' type='text/javascript'>
				top.openDialog('/stationary/jkt/webui/outbound/outbound_show.php?Code=$VoucherNo', 800, 470, 'Opening...', 'webui_outbound_outbound_show_$VoucherNo', 0, 'material_18.png', 0, 0);
				top.closeDialogOperations('#webui_outbound_outbound_request1');
		 </script>";

	//echo "<script language='javascript' type='text/javascript'>
      //  alert('Stok tidak ada');
		//top.closeDialogOperations('#webui_outbound_outbound_request1');
    //</script>";
}
 ?>