if you search google about HYIP and Ponzi schemes you would understand the system and how it works. just getting some new investors to pay for the older and till the moment the site operator determines they have got the most they can get..
Yeah, the bitcoin doubling sites are just Ponzi and scam, you give your money without any guarantee to get them back, there is no way you could double you bitcoin without risks it is too good to be true, if there was any people would be rich by now, if you want to double your bitcoin you could use them for gambling but like I said it involves a lot of risks.