I believe that if you want to double something it is logical to actually put it into a new technology that already has a existing customer base - like bitcoin. Unlike altcoins, it has a proven track record, and a lot more adopters and believers.
Bitcoin is nearly doubled in price this year, I can't see why it can't do so for next year as well.
Sometimes if we trying to double our money the more risk that ain't happened we taking up the risk more than the safest way we always want to take the profit without risking anything this is the problem. bitcoin is good to grow our bitcoin if he start buying bitcoin maybe his money is doubled right now.
Most people would really tend to go the the risky way of doubling money rather than thinking on the safe side. Actually theres no safe side on an investments since theres always a risk involve and thats really a normal thing but they do really differ on which investment would you do tend to put your money on. There are high risk and less risk but both have the chances of lossing money.