Emit warning if a name is duplicated in a module's export list

I'd find it helpful if pyflakes warned me that a module's `__all__` export list contains the same name more than once.

(The fact that `all_names` is passed through `set()` and yet this is not already checked for makes me wonder if there is a legitimate use for repeating a name in the export list or if this kind of check is just too expensive given the benefit. If so, apologies for the extra noise.)

Thanks for the great project.

Cheers,
Dave