How to not show on Label when change dropdownlist if duplicate found in ASP.Net

<asp:DropDownList ID="ddl" runat="server"  ClientIDMode="Static"  OnSelectedIndexChanged="ddl_SelectedIndexChanged"></asp:DropDownList>
<asp:Label ID="label" runat="server" ClientIDMode="Static" CssClass="field" Width="150px"></asp:Label>

<script type="text/javascript">
     $(function () {
         $("#ddl").change(function (e) {
             var itemExists = false;
             var txt = $("#label").val();
             e.preventDefault();
             $("#ddl option").each(function () {
                 if ($(this).text() == $.trim(txt)) {
                     itemExists = true;
                     alert('Item already exists');
                 }
             });
         });
     });
</script>