Before Adding Items to DropDown Lists check Existing Item using Contains Property
<pre lang="cs">DropDownList ddl= new DropDownList();
if(!ddl.Items.Contains(new ListItem("item")))
{
ddl.Items.Add(new ListItem("item"));
}
If you are calling DataBind() method make sure the List is Clear First
if(ddl.Items.Count>0)
ddl.Items.Clear();