Issue using dropdown lists with update panel

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ASP.NET

SQL

.NET

C#3.5

I am using 2 dropdown lists in parent child relationship and are present inside one update panel but when even i select some data from parent dropdown list, insted of filling the second dropdown list it just creates duplicate dropdown lists for both the dopdown list making 4 dropdownlists on the page.

My code is

<asp:UpdatePanel ID="up1" runat="server" UpdateMode="Always">
<ContentTemplate>
          <td valign="top">
                                                        <asp:DropDownList ID="ddlProject" runat="server" Width="105" AutoPostBack="true"
                                                            OnSelectedIndexChanged="ddlProject_SelectedIndexChanged">
                                                        </asp:DropDownList>
                                                    </td>
                                                    <td valign="top">
                                                        <asp:DropDownList ID="ddlSubProject" runat="server" Width="105" AutoPostBack="true"
                                                            OnSelectedIndexChanged="ddlSubProject_SelectedIndexChanged">
                                                        </asp:DropDownList>
                                                    </td>
                                                
</ContentTemplate>
 <Triggers>
<asp:AsyncPostBackTrigger ControlID="ddlProject" EventName="SelectedIndexChanged" />
</Triggers>
</asp:UpdatePanel>

Code is as folows

protected void ddlProject_SelectedIndexChanged(object sender, EventArgs e)
   {
       ClsCommon.FillDropDown(ddlSubProject, BALProjectUseCases.GetAllSubProjectByProject(Convert.ToInt32(ddlProject.SelectedValue)), DropDownSelectText, "FieldName", "id");

}

protected void ddlSubProject_SelectedIndexChanged(object sender, EventArgs e)
   {
       ClsCommon.FillDropDown(ddlUseCase, BALProjectUseCases.GetAllUseCasesByProject(Convert.ToInt32(ddlSubProject.SelectedValue)), DropDownSelectText, "FieldName", "id");
   }

public static void FillDropDown<T>(DropDownList ddlList, List<T> oList, string strSelectText, string strDataTextField, string strDataValueField) where T : class
   {
       try
       {
           if (oList.Count > 0)
           {
               ddlList.Items.Clear();
               ddlList.DataSource = oList;
               ddlList.DataTextField = strDataTextField;
               ddlList.DataValueField = strDataValueField;
               ddlList.DataBind();
               if (strSelectText != "")
                   ddlList.Items.Insert(0, new ListItem(strSelectText, "0"));
           }
           else
           {
               ddlList.Items.Clear();
               ddlList.Items.Insert(0, new ListItem(strSelectText, "0"));
           }

       }