Its called a Ponzi. If you want to double your coins without any work you could try gambling. At least you know your odds. Be carefull though the gambling section is full of ponzis as well. Many of the cloud mining services are also under suspect of beeing ponzis. Bitcoin is money, its not going to double without work/risk.

More about ponzis and ponzi "games" can be found here: https://bitcointalk.org/index.php?topic=927206.0