There's no easy cash even in bitcoins, But the ways to earn BTC are a little easier and can be done online, Doubling is a troubling word however since most of the sites that need a deposit and offer doubling are either extremely risky or scams, You might be better off working in Survey or Get paid to sites paying in BTC and accumulating it "BTC" overtime.
No site can double your coins in quick time, I mean its impossible to do, all those sites who promises to double your coins are 100% fraud and their main intention is only to scam people, nothing else.
even if by some miracle and some excellent busines on the back they could double the coins you invest there, they will never share such a big profit with their investors unless they are scammers.
also if they had a profitable business they would double their own coins and make millions.
Yeah if there is something like doubling bitcoins then no one will share their secret formula with others, instead they will keep on doubling their own coins.