I was bogged down for several hours dealing with this problem and did not find any helpful information on the forum - only one article seemed similar and it was indicated as a bug in 518 build (http://www.telerik.com/community/forums/thread/b311D-beamed.aspx)...however the issue I ran into happens even in the "Futures" build for Q2.
Basically if you have EnableHistory="true" or something else that might cause an AjaxRequest to be fired off before the page completes initialization phase you will get errors about double-registration. Specifically this seems to happen because the Telerik framework must be able to track what has been defined and what hasn't between Ajax requests so it knows what definitions it should return when an update panel fires off.
It would be very useful if Telerik could provide some mechanism to verify if page initialization has completed before firing off the ajax request. Sys.Application.get_isInitialized( ) perhaps would do the trick but more likely would need to use add_load method to change a bit flag allowing ajax requests to go through (perhaps queuing those initiated prior to load completing).
At any rate in my case I ended up fixing this by putting in my own checks to determine when the page was fully loaded before executing the request. Another thing others might find helpful is to use LoadScriptsBeforeUI="true" but there is still no guarantee that the init will finish before you make your ajax request unless you make sure of it yourself.
Oh...and if you're using something like:
| Sys.Application.add_load( |
| function( sender, args ) |
| { |
| if( !args.get_isPartialLoad( ) ) |
| doWork( ); |
| }); |
Make sure you have that check for partial load so you don't see your function firing constantly on the ajax updates.
Telerik...would love to hear from you guys in terms of something that will better coordinate what to load in an ajax call. Like perhaps you guys can register a list of everything that will be initialized "eventually" with the RadAjaxManager so that this kind of problem cannot occur.
Thanks!