I was frequently getting the "Multiple controls with the same ID 'ScriptManager' were found" error message. I corrected the error by doing as Ven Thompson suggested in his post earlier in this thread. I have further info to post regarding this solution. After I ran the resource file verifier, it listed 5 resource files with duplicate entries. When I examined the files in Visual Studio, I did not see the duplicates. I was able to locate and remove the duplicates using Notepad.

The duplicates were in 4 different modules: Events, Feedback, Links and Store. The duplicates were not in a consistant location in the files and it looked to me like the duplicates had perhaps been introduced over an extended period of time and have just now become a problem, perhaps because of a change somewhere in the DNN application.

There was a bug in the .NET 1.1 (VS2k3) resource editor that allowed duplicates to be created from time to time.  I found the "easiest" way to remove them was to download Lutz Roeder's resource editor, sort by name, and then you can visually see the dupes and remove them.  Obviously it would be better if the module developers themselves removed these that way when an update comes out we don't have to go remove them again ourselves.

http://www.aisto.com/roeder/DotNet/ -- Lutz Roeder's apps