How to not show on Label when change dropdownlist if duplica

Question

Following my title, I don't know why it not working. Please help me !. Please show code if need. here my code

<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>