Detect duplicated interface and avoid generating implementation

[arai-a]

Separated from https://github.com/binast/binjs-ref/pull/144

in the current spec, FunctionBody and ListOfStatement are same thing

typedef FrozenArray<Statement> FunctionBody;

but the code generator tries to generate implementation for both of them, resulting compile error because of duplication. in the patch in above PR, I added special case for FunctionBody, but we should replace it with more robust code.