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.
I also dont think there are much options available in bitcoin to double the amount,there are many but most of them are scam sites so founding any legit way in bitcoins is really very difficult.So better withdraw in fiat and start a business with that in real life.