This pesky error has been showing up as a runtime error in the Logmate output for several people having problems with the game: crash at startup, crash in editor, etc. It is suspected to be the cause of such problem but it is so difficult to fix that users generally end up in deleting whole routes or re-installing the game.
Actually the difficulty lies not in fixing the problem, which is self-evident, but in finding WHICH scenario GUID is duplicated, a bit of information that amazingly the error message doesn't provide.
You can now find out if you have duplicate scenario GUIDs, and if so which ones in which routes, in a few seconds by using a small script available here on the RWA web site.
See this tutorial in the RWA learning center.
EDIT: New simpler version with less confirmations.