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.
Currently the price of bitcoin start to increase and I have doubt that iy will once again double up in value so any person who will have their money in bitcoin will double up in a few months, and if someone want to double the amount of bitcoin then they should have to do trade.
I thought you would not be able to double your money by relying on price increases and in single trade? because prices will not rise twice as much. so I think you could double your money with a few tries trade. so you have to be patient in doubling your money