"The problem you experience is happening because you update a floating RadDock with AJAX. When dragging the dock you move it outside the update panel and this causes AJAX not to work properly as it attempts to recreate the dock at the place it was previously located and the dock is moved to the new place and stays there. In this way two docks with the same id appear on the page and this leads to an exception. Note that this is a common problem of controls which can be moved in the DOM tree.
A simple example which illustrates how to add/remove RadDocks via Ajax is available here: http://demos.telerik.com/aspnet-ajax/dock/examples/myportal/defaultcs.aspx Keep in mind that you should update all RadDockZones while all RadDocks are docked on every ajax call."