Doubling bitcoins is possible but its pretty hard, with websites its without any doubts not possible but that is already clear.
With trading there are some options but it also has some big risks to be honest.
Yes doubling bitcoin is essentially doubling money so its the same difficult maybe even more difficult because bitcoin you can do less with than fiat atm. you can try doubling by trading but it will be a long journey and risky.
It may be risky but if luck favors you than you can double it even in a quick time and I think that is the best way to double your coins as the power of making decision is in your hands.