Let me start with pointing you to the current documentation for the RadControls for ASP.NET AJAX, as the link you pasted if from the old, "Classic" suite's documentation which is no longer supported. Here begins the current version: http://www.telerik.com/help/aspnet-ajax/introduction.html
and here the RadWindow's: http://www.telerik.com/help/aspnet-ajax/window-overview.html
. Please keep in mind that if you reuse the same window adding handler more than once will naturally result in it being called multiple times. What you can do to avoid this is to remove the handler in the OnClientClose event handler itself.
An alternative solution is to declare two different RadWindows - one with the hander and one without it (or any other different configurations you may need) and use the one you need. You may find useful this article on the different ways to open a RadWindow: http://www.telerik.com/help/aspnet-ajax/window-programming-opening.html
Yet another option is to set the DestroyOnClose property of the manager to true so that each RadWIndow is disposed when it is closed. This will make sure you create a new instance each time and it will have only one handler attached (you can attach it only if need be, a simple if statement).
I would also advise that you examine the current client-side API of the RadWindow here: http://www.telerik.com/help/aspnet-ajax/window-programming-radwindow-methods.html
the Telerik team
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