Who knows how to double Bitcoin ... by trading in stock or else ... eg doubles as those who say that any call of BTC doubles in 3 4 hours?
Double money in 3-4 hours can't be legit. I see some people talking about high profits trading, but it's not easy and certain. There is some risk involved and most traders don't make this high profit in so short time intervail.
Furthermore, I don't suggest you to invest on any site which offers high BTC returns so fast and easily.