I believe you are creating a new RadWindow in every button click and you are setting its VisibleOnPageLoad proeprty to true. If so - you are getting the expected behavior - you add many control instances to the page and you make each of them show when the page reloads.
If you want to show only one after each postback I strongly suggest you carefully examine the following sticky forum thread on the matter: http://www.telerik.com/community/forums/aspnet-ajax/window/opening-radwindow-from-the-server.aspx
the Telerik team
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