1'st rule of trading; sell for more than you paid + fees every time. A doubling of your money can take 6 trades in one minute or 3 months, that depends on the market being wild enough while you look at the ticker.
Trading is not hard, just take luck out of your calculations meaning don't trade on what you think will happen, BUT ON WHAT HAPPEND! Very important that last point, then you wont ever loose money on a trade.
Market trading is probably the safest way Why do you look to directly DOUBLING the Bitcoins? Take it slow and steady on the market, do your research and then buy/sell based on the recent conditions. It takes a little patience to get the knack of how it works. Best of luck to you anyway..