In bitcoin we do not have method than we can double our bitcoin safely we always need to take a risk to earn profit but we can reduce the risk we have by investing multiple, bitcoin has more method to invest then we need to try it all to earn more profit.
Well there is but that is the old method and that are with trading and you could also invest into bankroll casino's that are the legit ways.
Because many are thinking to invest that money on hyip's , cloud mining or any ponzi schemes but that is just like you are donating money to them.
It is better the hard way, slowly but surely and you have assurance that your money is secured.