Hi, I just tried creating a simple sample with Modalpopup extender and Caledarextender. My design is like below.

  <asp:UpdatePanel ID="pnlUpdate" runat="server">
        <ContentTemplate>
            <asp:Panel ID="pnlSample" runat="server" CssClass="frame">
                <asp:TextBox ID="txtOSDate" runat="server"></asp:TextBox>
                <asp:TextBox ID="txtDate" runat="server"></asp:TextBox>
                <asp:CalendarExtender ID="calDate" runat="server" TargetControlID="txtOSDate">
                </asp:CalendarExtender>
                <asp:CalendarExtender ID="calOSDate" runat="server" TargetControlID="txtDate">
                </asp:CalendarExtender>
                <asp:Button ID="btnCancel" runat="server" Text="Cancel" />
            </asp:Panel>
            <asp:Button ID="btnHiddenSample" runat="server" Style="display: none;" />
            <asp:Button ID="btnSample" runat="server" CssClass="Button" Text="New Sample" />
            <asp:ModalPopupExtender ID="mpeSample" runat="server" TargetControlID="btnHiddenSample"
                CancelControlID="btnCancel" PopupControlID="pnlSample" DropShadow="false" BackgroundCssClass="modalBackground">
            </asp:ModalPopupExtender>
        </ContentTemplate>
    </asp:UpdatePanel>

When i run the sample and click the "New Sample" button, i get javascript error as "Error: Sys.InvalidOperationException: Two components with the same id 'ctl00_MainContent_calDate' can't be added to the application.' can't be added to the application." I checked the rendered HTML using DeveloperTools and i still found the asp Panel recreated. Someone please advice me what is the actual problem?

PS:When i comment the Calendar Extender, i dont get the error.

Regards, Lakxman Kumar C