"If you think that something is too good to be true, then it probably is."
Don't mess up with such systems, they are only doubling their own bitcoins, not yours.
These Ponzis/HYIPs have definitely benefited a lot from crypto-currencies.
This is unfortunate but true, the first time I saw something like that, the scammers were using egold but now they are using bitcoin due to the inability of chargeback a transaction with bitcoin, so the lesson here is we have to be even more careful with our bitcoin than with fiat.