I Used Clear Selection But Why I Get Error :Cannot Have Multiple Items Selected In A Dropdownlist

List<Country_tbl> listOfCountry = MyDataBase.Country.Liste.Complete;//get list of Country from database

controls.DropDownList.Bind(listOfCountry, ddlCountry );//binding list to ddl
if (Request.Cookies["SelectedCountry"] != null)
{
      string Countryid=Request.Cookies["SelectedCountry"].Value;
      controls.DropDownList.SelectItem(ddlCountry, Countryid);//select item from ddl that have Value==countryid
      List<State_tbl> listOfState = MyDataBase.State.list(Countryid);//BreakPoint: here i check ddlCountry it have One Selected Value
      controls.DropDownList.Bind(listOfState , ddlState);
      //if (Request.Cookies["SelectedState"] != null)
      //{
      //    string Stateid=Request.Cookies["SelectedState"].Value;
      //    controls.DropDownList.SelectItem(ddlState, Stateid);
      //    List<City_tbl> listOfCity = MyDataBase.City.list(Cityid);
      //    controls.DropDownList.Bind(listOfCity , ddlCity);
      //      ///...
      //}
}// BreakPoint: in this point i found that ddlCountry have two Selected value

public static void Select(DropDownList ddl, string Value)
{
    ddl.ClearSelection();
    ddl.Items.FindByValue(Value).Selected = true;
}