repeat items in dropdownlist after postback

I have a dropdownlist which is like below:
<asp:DropDownList ID="ddlUser" runat="server" CssClass="commonText" AutoPostBack="True"
DataSourceID="ObjectDataSourceEmployeeNames" DataTextField="UserName" DataValueField="UserID"
AppendDataBoundItems="True" OnSelectedIndexChanged="ddlUser_SelectedIndexChanged" Width="200px&q ...