求助

<style>
.cls1
{
position:absolute;
left:250px;
top:89px;
width:216px;
height:72px;
z-index:1;
}
.cls2
{
position:absolute;
left:250px;
top:89px;
width:95px;
height:18px;
z-index:2;
}
input
{
font-size: 12px;
padding-top: 2px;
padding-left: 2px;
width:127;
}
</style>
<script>
function addOption(pos)
{
if(event.keyCode==13) {
var select_obj = document.getElementById("myselect");
var text_value   = document.getElementById("test").value;
if (select_obj.length) {
for (var i=0;i<select_obj.length;i++) {
// 如果已经存在，不添加，直接退出
if (select_obj.options[i].text==text_value)
return false
}
}
var the_option= new Option(text_value,text_value);
select_obj.add(the_option);
select_obj.selectedIndex = select_obj.options.length-1
}
}
</script>
<div class="cls1" style="clip: rect(3 280 21 110)"> 
<select name="myselect" style="width:127" onchange="document.getElementById('test').value=this.value">
</select>
</div>
<div class="cls2">
<input type="text" name="test" onkeydown="addOption()" size="20" style="width: 127; height: 23">
</div>
不会重复输入