The refresh button RadWindow offers can merely invoke the window.reload() method of the content iframe. At this point the browser takes over and things depend on the browser and ASP from then on. If a postback has already occurred on the page the most likely effect is that a browser prompt will appear that will ask the user if the last POST should be repeated.
Some browsers attempt to keep forms data and this may also have taken place.
What I can suggest is using the OnClientCommand event and replace the default reload action with replacing the URL of the page with a random querystrgin, which is what the ReloadOnShow feature of the control does. Here is an example how you can use the methods RadWindow already has:
changedUrl = sender._getReloadOnShowUrl(sender.get_navigateUrl());
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