Please give idea to solve this problem . I am unable to solve this .
Error is
Only one instance of a ScriptManager can be added to the page.
Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code

<%@ Page Language="C#" MasterPageFile="~/Admin.master" AutoEventWireup="true" CodeFile="last_try.aspx.cs" Inherits="last_try" Title="Untitled Page" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<asp:Content ID="Content1" ContentPlaceHolderID="ContentPlaceHolder1" Runat="Server">
    <table style="width: 100%">
        <tr>
            <td>
                <asp:ScriptManager ID="ScriptManager1" runat="server">
                </asp:ScriptManager>
            </td>
        </tr>
        <tr>
            <td>
                <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
                </asp:ToolkitScriptManager>
                <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="open" CancelControlID="cancel">
                </asp:ModalPopupExtender>
                &nbsp;</td>
        </tr>
        <tr>
            <td>
                <asp:Button ID="open" runat="server" Text="open" onclick="open_Click" />
                <asp:Button ID="cancel" runat="server" Text="cancel" />
            </td>
        </tr>
    </table>
</asp:Content>