But, you will get this message when you have loaded the same file twice, too. This happens always, if you have files with case letters (i.e. File.h/File.cpp) and use the swap header/source function (F11). Sometimes, if the compiler find an error message and loads the source file, you will get a second copy of the source file. It seems, that c::b does not recognizes all loaded files.
So, next time you will get this message, have a look in the Window menu.
And yes, this is a very nice IDE!
bye,
stefan