Under some unknown conditions, all MVC pages fail with the message "An item with the same key as already been added". This issue has been reported on stackoverflow and forums.asp.net, see
All MVC pages fail with the message an item with the same key has already been added
. I have had it happen 4 times in the past 6 months, and when this conditions happens, all MVC pages are unusable until an appPool reset. WebForm pages continue to operate
without issue. This issue has spanned a migration to MVC3 to MVC4 and does not appear to be addressed in the latest nightly builds.