Really difficult to double bitcoins. The best way to double the bitcoins is by playing trading. Trading can make us get a great advantage when choosing the right time to sell and buy the coin.
I agree, its hard to double our bitcoins and in trading it can be possible just be patient and learn how to trade accordingly. it can take weeks or months but for me its way better than other investments.
Agree it is very hard if you are new and you dont have enough bitcoin to invest more but trading is the best way to double it into long period of time. You can double it in long term trade! That is the only one that it can double oh i forgot also the short term but it really takes time on it buy and sell