http://en.wikipedia.org/wiki/Ponzi_scheme "A Ponzi scheme is a fraudulent investment operation that pays returns to its investors from their own money or the money paid by subsequent investors, rather than from profit earned by the individual or organization running the operation."
I am doing some research recently regarding this topic, it is a bit complex, it depends on the long term sustainability of the return
You save your money in the banks and receive 2% interest per year. This could also be a ponzi scheme, since the banks could get more and more deposit each year from new depositors and pay you interest with their deposits
But there is a limitation on how big that interest can be, that's the growth of available new deposits, and ultimately limited by the money supply expansion speed. If money supply do not expand more than 5% per year, it is impossible for banks to give you a return of higher than 5% per year constantly
"Profit earned by the individual or organization running the operation", this term is unclear, what is an operation? Isn't providing a deposit service an operation? This service itself can draw in more and more deposit each year due to expansion of money supply, and if the money supply expansion is constant, then this 2% profit model can be maintained forever
People usually define very high return to be a ponzi scheme, since it is not sustainable, will eventually run out of new depositors and late investors will lose their money
So it all depends on the sustainability. If this return is long term sustainable, then it will be a financial innovation instead of a ponzi scheme
Back to OP's question, a 2X return in bitcoin in one year, is it long term sustainable? We all know that bitcoin's money supply is decreasing each year, so that is definitely not sustainable, so it is a ponzi scheme for sure