Ajax Control Tool Kit(3.0)& asp.net(3.5) Problrm Getting Exception

I have a problem
 When i was using Ajax Control Tool Kit 1.0 that time i was getting no source code available further ...... that kind of error was comming.

But now I changed my Ajax Control Tool kit to 3.0 now i am getting exception
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."
Exception Details:
System.Web.HttpException: Multiple controls with the same ID '_header' were found. FindControl requires that controls have unique IDs.
Stack Trace:

[HttpException (0x80004005): Multiple controls with the same ID '_header' were found. FindControl requires that controls have unique IDs.]
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +273
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls) +320
   System.Web.UI.Control.EnsureNamedControlsTable() +61
   System.Web.UI.Control.FindControl(String id, Int32 pathOffset) +222
   System.Web.UI.Control.FindControl(String id, Int32 pathOffset) +255
   System.Web.UI.Control.FindControl(String id) +12
   AjaxControlToolkit.ExtenderControlBase.FindControlHelper(String id) in C:\Users\swalther\Projects\AspNetAjax\Releases\30930\AjaxControlToolkitSource\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:181
   AjaxControlToolkit.ExtenderControlBase.FindControl(String id) in C:\Users\swalther\Projects\AspNetAjax\Releases\30930\AjaxControlToolkitSource\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:209
   System.Web.UI.ExtenderControl.RegisterWithScriptManager() +72
   System.Web.UI.ExtenderControl.OnPreRender(EventArgs e) +19
   AjaxControlToolkit.ExtenderControlBase.OnPreRender(EventArgs e) in C:\Users\swalther\Projects\AspNetAjax\Releases\30930\AjaxControlToolkitSource\AjaxControlToolkit\ExtenderBase\ExtenderControlBase.cs:367
   System.Web.UI.Control.PreRenderRecursiveInternal() +80
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.WebControls.WebParts.WebPart.PreRenderRecursiveInternal() +42
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Control.PreRenderRecursiveInternal() +171
   System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint) +842

Please send some solution on this problem I am searching for almost 2 days for it.
Thanks.