This question asks whether a traveler looking through a telescope will see his earthbound twin moving in slow motion. It is closed as a duplicate of this question, which asks whether a traveler moving away from earth will hear earth-originated telephone conversations in "slow motion".
It wouldn't surprise me at all if the closed question were a duplicate of something, but it's not a duplicate of the currently alleged duplicate. That's because the answer to the closed question depends on whether the traveler is moving toward or away from earth, whereas the alleged duplicate addresses only the (less interesting) case where the traveler is moving away.
So in particular, readers are told that "This question already has an answer here", and then directed to a site where the more interesting part of the question is not answered.
Again---I wouldn't be the least bit surprised if there's a real duplicate somewhere, but it would be good if readers were directed to that real duplicate.