AJAX :: Server Code Executing Twice With Control Toolkit

I am using following server side code to save the data from ASP.NET Control toolkit Editor to the database It follows with the HTML Code The problem is that, data is saved successfully. But it is saved twice. Main problem is this server side event is fired twice. How can i prevent it from firing twice?

Server side code
Imports System.Data.SqlClient
Partial Class Administrator_Calendar
Inherits System.Web.UI.Page
Protected Sub BtnAdd_Click(ByVal sender As Object, ByVal e As System.EventArgs) Handles BtnAdd.Click
Try
Dim dbobj As New db
Dim i As Integer = 0
Dim rdr As SqlDataReader = dbobj.selectQuery("select max(srno) from calendar")
If rdr.HasRows = True Then
rdr.Read()
i = rdr.Item(0)
End If
rdr.Close()
dbobj.insertQuery("insert into calendar values (" & i + 1 & " , '" & Me.Txt_dt.Text & "' , '" & Me.Editor1.Content & "')")
dbobj = Nothing
Catch ex As Exception
End Try
End Sub
End Class
HTML CODE
<%@ Page Language="VB" MasterPageFile="~/MasterPages/Admin_AftrLogin.master" AutoEventWireup="false"
&nbsp;&nbsp; &nbsp;CodeFile="Calendar.aspx.vb" Inherits="Administrator_Calendar" Title="Excel Crop Care Ltd. / Administration Section / Manage Calendar..." %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit.HTMLEditor"
&nbsp;&nbsp; &nbsp;TagPrefix="cc2" %>
<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="Server">
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder2" runat="Server">
<div>
<cc1:TabContainer ID="TabContainer1" runat="server" ActiveTabIndex="0">
<cc1:TabPanel runat="server" HeaderText="Selected Dates" ID="TabPanel1">
<ContentTemplate>
<asp:GridView ID="Gridview1" runat="server" AutoGenerateColumns="False" DataKeyNames="srno"
DataSourceID="SqlDataSource1">
<Columns>
<asp:CommandField ShowDeleteButton="True" ShowEditButton="True" ShowInsertButton="True">
<ItemStyle Width="20%" />
</asp:CommandField>
<asp:BoundField DataField="srno" HeaderText="No." ReadOnly="True" SortExpression="srno">
<ItemStyle Width="10%" />
</asp:BoundField>
<asp:BoundField DataField="date" HeaderText="Date" SortExpression="date">
<ItemStyle Width="30%" />
</asp:BoundField>
<asp:BoundField DataField="description" HeaderText="Description" SortExpression="description" />
</Columns>
</asp:GridView>
<asp:SqlDataSource ID="SqlDataSource1" runat="server" ConnectionString="<%&#36; ConnectionStrings:DataConnectionString %>"
SelectCommand="SELECT * FROM [calendar]" UpdateCommand="Update calendar set date=@date,description=@description where srno=@srno"
DeleteCommand="Delete from calendar where srno=@srno"></asp:SqlDataSource>
</ContentTemplate>
</cc1:TabPanel>
<cc1:TabPanel ID="TabPanel2" runat="server" HeaderText="Add New">
<ContentTemplate>
<table>
<tr>
<td width="30%">
Date :
</td>
<td>
<asp:TextBox ID="Txt_dt" runat="server" Height="16px" Width="170px" CausesValidation="True"
Font-Names="Verdana" ForeColor="#666666"></asp:TextBox>
<asp:ImageButton runat="Server" ID="Image1" ImageUrl="../Images/Calendar_scheduleHS.png"
AlternateText="Click to show calendar" /><br />
<cc1:CalendarExtender ID="Txt_dt_CalendarExtender" runat="server" DaysModeTitleFormat="dd/MM/yy"
Enabled="True" Format="dd/MM/yy" TargetControlID="Txt_dt" PopupButtonID="Image1">
</cc1:CalendarExtender>
<cc1:TextBoxWatermarkExtender ID="Txt_dt_TextBoxWatermarkExtender" runat="server"
Enabled="True" TargetControlID="Txt_dt" WatermarkText="DD/MM/YY">
</cc1:TextBoxWatermarkExtender>
<cc1:MaskedEditExtender ID="Txt_dt_MaskedEditExtender" runat="server" MaskType="Date"
CultureDateFormat="dd/MM/yy" Enabled="True" TargetControlID="Txt_dt" Century="2000"
UserDateFormat="DayMonthYear" Mask="99/99/99">
</cc1:MaskedEditExtender>
</td>
</tr>
<tr>
<td colspan="2">
<asp:Label ID="Lbl_msg" runat="server" Text="Enter description in the editor below :"></asp:Label>
</td>
</tr>
<tr>
<td colspan="2">
<cc2:Editor ID="Editor1" runat="server" Height="350px" />
</td>
</tr>
<tr>
<td colspan="2" align="center">
<asp:Button ID="BtnAdd" runat="server" Text="Add" OnClick="BtnAdd_Click" Width="75px"
CssClass="btn" />
<asp:Button ID="Btn_Rem" runat="server" Text="Remove" OnClick="BtnAdd_Click" Width="75px"
CssClass="btn" />
</td>
</tr>
</table>
</ContentTemplate>
</cc1:TabPanel>
</cc1:TabContainer>
</div>
</asp:Content>