Go to a online casino supporting bitcoin payment, buy some chips and go to a single zero roulette game, bet all on RED: 47.3% chance to double it 
If you are looking to double your bitcoin in the quickest way then I would suggest you to go for gambling sites where you can gamble on any of your favorite sports which would help you to double your bitcoin in very less span of time but there is a risk but remember :"High Risks High Return"
