required field validator fails second time for listbox control

See more:

C#

ASP.NET

HI,
I have a problem to add required field validator for listbox.
when I fill the listbox with the values and submit the record it works perfectly. before submiting the record it checks for the required field in the listbox.
But after submitting the record if I try to submit another record even if the listbox has the values it again ask the required field.

<td width="40%" align="center">
                                    <asp:ListBox ID="lstSelectedStudents" runat="server" Width="150px" Height="120" SelectionMode="Multiple"></asp:ListBox>
                                </td>
                                <td align="center" width="10%">
                                    <%--<asp:RequiredFieldValidator ID="Rfv_lstSelectedStudents" runat="server" ControlToValidate="lstSelectedStudents" ErrorMessage="Select student list" Text="*" ValidationGroup="Vg_Activity" ForeColor="Red" Display="Dynamic"></asp:RequiredFieldValidator>--%>
                                    
                            </tr>

Please help...