How set distinct value to dropdown list

private void FillInt()
{
using (SPSite site = SPContext.Current.Site)
{
using (SPWeb web = site.OpenWeb())
{
web.Lists.IncludeRootFolder = true;
SPList ChartList = web.Lists["Organization Charts"];

//query.Query = "USA";


SPQuery query = new SPQuery();
query.Query = "";
SPListItemCollection listItems = ChartList.GetItems(query);
foreach (SPListItem listItem in listItems)
{
FillDropDownList(listItem, ddlGroup, "field");
}
}
}
}
private void FillDropDownList(SPListItem listItem,DropDownList ddlTemp, string fieldName)
{
if (null == ddlTemp.Items.FindByText(listItem[fieldName].ToString()))
{
ListItem ThisItem = new ListItem();
ThisItem.Text = listItem[fieldName].ToString();
ThisItem.Value = listItem[fieldName].ToString();
ddlTemp.Items.Add(ThisItem);
}

}