AJAX Firing OnUnload Twice

Question

All, thanks in advance.

I'm using the ASP.NET AJAX Control Toolkit to create 2 CascadingDropDowns. The problem is the base OnUnload is firing twice when I load the page. Any idea how to stop this? See code snippets below.

Update: I've determined a workaround but if anyone knows of a way to suppress, I would be interested to hear!

<%@ Page Language="C#" AutoEventWireup="true" 
  CodeFile="dataentry.aspx.cs" 
  Inherits="dataentry" 
  EnableEventValidation="false" %>

<ajaxToolkit:ToolkitScriptManager EnablePartialRendering="true" 
  runat="server" 
  ID="ScriptManager1" />

<asp:DropDownList ID="drpDate" runat="server"></asp:DropDownList>
<ajaxToolkit:CascadingDropDown ID="CascadingDropDown1" runat="server" 
  TargetControlID="drpDate" 
  Category="Date" 
  PromptText="Please select a date" 
  ServicePath="SchedulerService.asmx" 
  ServiceMethod="GetSchedulerDates" 
  UseContextKey="true" />

<asp:DropDownList ID="drpTime" runat="server"></asp:DropDownList>
<ajaxToolkit:CascadingDropDown ID="CascadingDropDown2" runat="server" 
  TargetControlID="drpTime" 
  ParentControlID="drpDate" 
  PromptText="Please select a time" 
  ServiceMethod="GetSchedulerTimesForDate" 
  ServicePath="SchedulerService.asmx" 
  Category="Time" 
  UseContextKey="true" />