However, I can point out that Vader obviously knows who Luke is long before the Emperor scene. Remember he says, 'That's it. The Rebels are there. And I'm sure Skywalker is with them.' Therefore his asking the Emperor, 'How is that possible?' seems redundant. Why is he asking about something that he already knows the answer to? This seem to fall into the area of godd idea but bad execution. This doesn't seem like it was thought out properly.
However, I can point out that Vader obviously knows who Luke is long before the Emperor scene. Remember he says, 'That's it. The Rebels are there. And I'm sure Skywalker is with them.' Therefore his asking the Emperor, 'How is that possible?' seems redundant. Why is he asking about something that he already knows the answer to? This seem to fall into the area of godd idea but bad execution. This doesn't seem like it was thought out properly.