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.
Trading is a legitimate way to double your bitcoins but you have to had a good patient as that will not happen overtime, if you are looking for the fast way to double your bitcoins then trading is not the answer but gambling, really feel good when we are lucky in gambling as in a matter of seconds we can double our money.
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.