Really difficult to double bitcoins. The best way to double the bitcoins is by playing trading. Trading can make us get a great advantage when choosing the right time to sell and buy the coin.
Yes that is the best place at a moment to double your coins with good strategy and skills you can expect that result over a period of time but you also need to have great level of patience.
You are correct gambling is the fastest way on doubling your bitcoin but i wouldnt recommend this because instead of doubling your money you would surely lose your money instead but still theres a possibility to double in no time but as others said it has higher risk on losing it. Trading would be the best way but it would take time.
Agree bitcoin is one of the fastest way but it is very very risky at all because if you are planning to double your bitcoin but instead your bitcoin will be gone forever! haha so if you want to double you bitcoin in fastest way then try gambling then you will loose your bitcoin easily
Try trading dude.