Doesn't correctly handle duplicate docstring

[heejongahn]

If an IDL code uses docs syntax and @docs annotation for the same declaration at a time it currently doesn't provide any useful error message but ... (from #72)

Actually, Nirum does not provide any error message at all in this case. It just works. Which is, sadly, not a desired behavior. The parser might be just overriding the first docstring and applying the second one, instead of reporting a syntax error.