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
I would suggest staying away from ponzis. 90% of them have been scams , so instead of doubling there is a higher probability you will lose everything.
Just use some of the skills you have to earn, or if you wish to take the risk and gamble, just do that.