offer a service or sell something to people, money doesnt come easy man you gotta work for it
Offering a service is like a job though, OP talk about doubling bitcoin. I think gambling is the easiest way to doubling your bitcoin, it's really risky but it's the easiest way.
You can say gambling is easy money but it is not a sure thing. You can also easily lose money in there. So instead of doubling the bitcoin you got, you'll be losing them. It is better to work hard and earn bitcoin the hard way. It is safe, sure and no chance of losing. If you have a stable gig in bitcoin, you'll doubling what you have in no time.
Gambling will allow you to double your coins once, twice or thrice but you cannot expect every time, its better to work hard on it and make honest efforts to double it in long run.