There are many ways on how you can earn free money and to double your bitcoins. Buy and resell will be the best option you can choose if you're going to ask me. Avoid HYIPS and doubler site as it is really not recommended if you want to earn online, it will not help you.
It's not included in the list of a way the doubling bitcoin, it's included in trading section. doubling it means you can made a 100% a profit at the short time.
But i never believe it's will gonna happen in trading will get a 100% of your profit. the suitable way is gambling but they have a high risk maybe it's equal with the profit you can get.