We have seen lots of doppelgängers in season 2 of the Flash. Not always the same name, not always good. We saw an example with Cisco and Caitlin where on Earth 2 they were Evil and Caitlin was actually a meta human.
Earth 2 is just one of infinite possible Earths. But Earth 2 is where Zoom is so we assume The Flash does find a way to get back there to get vengeance and maybe rescue the man in the iron mask.
Now we have just heard that Arrow star Katie Cassidy will guest-star as Black Siren on Earth-Two. She plays Laurel Lance in The Arrow.
She will be seen in the episode of The Flash airing May 17, confirmed by Arrow executive producer Marc Guggenheim at a Monday press screening
Black Siren is the only female member of the Justice Guild and is the Golden Age version of Black Canary, see more about her here: