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.
We should not think about doubling the bitcoins because bitcoin is also money and how can you double your money,yes you can but initially we should think only about achieving some from with those bitcoins and then with out work and luck it make be possible to achieve great profit from them or even double them.