where can I double\multiply bitcoins?
You can always gamble it away.
There is no magic formula to double it. Some invest in altcoins and try to make a quick profit, others trade and try the same. All of these methods carry an inherent risk so be careful.
Gambling is really just a horrible idea.
One think OP needs to think of with BTC is it's a form of money. So free/get rick quick is not going to happen. To double you would have to do risky investments (alt coins, loans, etc) unless you are willing to give it a reasonable time.