I now explain you because this can be sadly assimilated to a Ponzi scheme.
Imagine I'm depositing 0.5BTC.
To double my investment (less fee) somebody else will have to deposit 0.501BTC as well.
I'll then get 1BTC and the 0.001 fee will go to the website owner.
The person who deposited 0.501BTC will then wait for the next people depositing 0.501BTC.
But when no more new people will be available (the world's population is not infinite, and so the BTC supply), the last ones participating will wait indefinitely, possibly forever.  And there'll be no possibility of refund, simply because the credit will be exhausted.
It's true that people who already played could redeposit their BTC to unlock the other people funds.  But they would then lose their previous earnings.  At last, only the first ones participating will actually profit from this scheme, and the website owner (even though in minimal part).
Believe me, there's no way to indefinitely multiply money.