why button click insert data twice times

Protected Sub Button2_Click(sender As Object, e As System.EventArgs) Handles Button2.Click

      Dim rowIndex As Integer = 0
      Dim sc As New StringCollection()
      Dim sc1 As New Date
      If ViewState("CurrentTable") IsNot Nothing Then
          Dim dtCurrentTable As DataTable = DirectCast(ViewState("CurrentTable"), DataTable)
          Dim drCurrentRow As DataRow = Nothing
          If dtCurrentTable.Rows.Count < 10 Then
              For i As Integer = 1 To dtCurrentTable.Rows.Count
                  'extract the TextBox values

                  Dim box5 As TextBox = DirectCast(Gridview3.Rows(rowIndex).Cells(1).FindControl("TextBox5"), TextBox)


                  Dim box7 As Date = Convert.ToDateTime(box5.Text)


                  Dim myConn As New SqlConnection
                  Dim myCmd As New SqlCommand
                  myConn.ConnectionString = ConfigurationManager.ConnectionStrings("mydatabase").ConnectionString
                  Dim cmd As String
                  cmd = "Insert into Date values (@date) "
                  myCmd.CommandText = cmd
                  myCmd.CommandType = CommandType.Text

                  myCmd.Parameters.Add(New SqlParameter("@date", box7))


                  myCmd.Connection = myConn
                  myConn.Open()
                  myCmd.ExecuteNonQuery()
                  myCmd.Dispose()
                  myConn.Dispose()
                  rowIndex += 1
              Next


          End If
      Else
          '   lblMessage.Text = "Cannot save as there no information recorded"
          MsgBox("failed")
      End If
  End Sub

<asp:GridView ID="Gridview3" runat="server" AutoGenerateColumns="False" 
                Height="89px" ShowFooter="True" Width="303px">
                <Columns>
                    <asp:BoundField DataField="RowNumber" HeaderText="No of Available:" />
                    <asp:TemplateField HeaderText="New Date Available :">
                        <ItemTemplate>
                            <asp:TextBox ID="TextBox5" runat="server" />
                            <AjaxToolkit:CalendarExtender ID="calExtender6"  runat="server" 
                                Format="dd/MM/yyyy" OnClientDateSelectionChanged="CheckDateEalier" 
                                TargetControlID="TextBox5" />
                        </ItemTemplate>
                        <FooterStyle Height="22px" HorizontalAlign="Right" />
                        <FooterTemplate>
                            <asp:Button ID="ButtonAdd" runat="server" onclick="ButtonAdd_Click" 
                                Text="Add New Row" />
                        </FooterTemplate>
                    </asp:TemplateField>
                </Columns>
            </asp:GridView> 

Protected Sub ButtonAdd_Click(sender As Object, e As System.EventArgs)
      AddNewRowToGrid()
  End Sub