Well, if you know a Robin, then you're probably from an Earth. But I know at least fifty people from Earths. Some of them share the same common knowledge, some of them just share geography. Some of them have everything the same, some of them have just one or two things that are different. [Katara's eyes were compassionate.] So there might be two Robins here, but it's possible that neither one of them could be the Robin you know - just a lot like him.