I think you the caller was wrong to pass in [f, f] in the first place.

In asyncio, the argument is converted into a set before using it, but there's still a bug if you pass it a list containing two references to the same coroutine -- it gets wrapped in two separate Futures. I think the better behavior is to convert to a set first and then map coroutines to Futures.

So concurrent.futures should also convert to a set first.