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.
Trading is the only way to double your coins, but before you enter into it you should make sure that you are aware about the market and latest trends related to it.
Learning those things is not really that hard, anyone can learn how to do trading if he is really interested in it, all you will have to do is to read some articles about trading altcoins and then watch some news about the markets and you are good to go.