I think its hardly impossible to double your investment by investing in any cloud mining farms.Even you would have to wait for six months to recover your investment.Any other investment would also not help you to double your bitcoins.Investing in any altcoin and trading might help you but its highly risky.But trading is the only legit source nowadays to earn more bitcoins.
It was not hardly impossible,but it was impossible.
Cloud mining is not profitable these year,if we're in early 2014ish,we might be can double our investment but not right now.the problem is the difficulty and also when you buy a contract today what will you get today and tommorow will be different.plus we need to pay a fee which is very high.some people (or even many people) complained that their contract no longer profitable.
I prefer to trade other than cloud mining,but i do not recommend to trade on localbitcoin because my account get hacked multiple times and i do not know what is going on.from my observation,many phising website came up when i tried to login on localbitcoin.