HttpException Multiple controls with the same ID 'ctrl0' were found

Hi all,

I am getting the following error in my website randomly.

Exception:

Exception information: 
    Exception type: HttpException 
    Exception message: Multiple controls with the same ID 'ctrl0' were found. FindControl requires that controls have unique IDs. 

Stack Trace:

 Stack trace:    at System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls)
   at System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls)
   at System.Web.UI.Control.FillNamedControlsTable(Control namingContainer, ControlCollection controls)
   at System.Web.UI.Control.EnsureNamedControlsTable()
   at System.Web.UI.Control.FindControl(String id, Int32 pathOffset)
   at System.Web.UI.Control.FindControl(String id, Int32 pathOffset)
   at System.Web.UI.Control.FindControl(String id)
   at AjaxControlToolkit.ExtenderControlBase.FindControlHelper(String id)
   at AjaxControlToolkit.ExtenderControlBase.FindControl(String id)
   at System.Web.UI.ExtenderControl.RegisterWithScriptManager()
   at System.Web.UI.ExtenderControl.OnPreRender(EventArgs e)
   at AjaxControlToolkit.ExtenderControlBase.OnPreRender(EventArgs e)
   at AjaxControlToolkit.HoverMenuExtender.OnPreRender(EventArgs e)
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Control.PreRenderRecursiveInternal()
   at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint)

It seems to be that this error occurs while pre-render happens in HoverMenuExtender(I judged this by seeing the stack trace).