where can I double\multiply bitcoins?
Go in this section :
https://bitcointalk.org/index.php?board=56.0And go n Investor-based Games but be aware, you invest on your own risk and every site will run away there , they are all ponzi's , it;s just about your judgement and timing , though i would recommend not to use them and just learn and earn
Never try invest your money in "Investor-based Games", it's much more more dangerous than gambling & you only got small profit
You better go this section :
GamblingIf you know how to gambling & not greedy, you will get a lot of profit
You also can try trading using trading bot too