BC30560: 'CalendarExtender' is ambiguous in the namespace 'AjaxControlToolkit'

I converted an old web site project to web form project and updated  AjaxControlToolKit to v15.1,  followinging instruction for AjaxControlToolkitupgrade. (https://ajaxcontroltoolkit.codeplex.com/wikipage?title=Upgrade%20your%20project%20to%20AjaxControlToolkit%20v15.1) 

Now I got BC30560 error for CalenderExtender.   I searched and tried many suggestions in the web and  removed and added reference to make sure there are no multiple  AjaxControlToolKit references. but still  could not resolve the issue

I appreciate if  someone here can shed some light on the problem

Please note if I create a new web form project, added a page with  AjaxControlToolkit CalenderExtender, it works fine.. But if I added a new page in the converted project, I saw the error.

Thanks