ถ้าต้องการตัดค่าที่ซ้ำกันออก จาก drop down list ต้องทำไงครับ

$select_book = "1, 3, 5";

<?php
$sqlw="select * from tbl_workdata where wd_status=1 and wd_id not in ($select_book)"; 
$dataw=mysql_query($sqlw); 
while($roww=mysql_fetch_array($dataw)){ 
	$wd_id = $roww['wd_id'];
	$wd_name = $roww['wd_name'];

	$sql_select="Select * from tbl_goods_proc where gs_id='$gs_id' " ;
	$rs = mysql_query($sql_select); 
	while($r = mysql_fetch_array($rs)){
		$ck=$r['gp_name'];
	}
	if($wd_id !=$ck) { 
	?>
	<option value ='<? echo $wd_id; ?>'><? echo $wd_name; ?></option>
	<?
	}
}
?>