I forgotten a site where I've invested only 1mbtc and got 8mbtc. If anyone knows about it please let me know. also similar sites like that will do. Thanks for reading.
in my opinion there no such sites but may some online casino provide the more profit then double at win of bet .
but it all depends upon the multiplier you choosed before the start gambling .
secondly all sites in gambling category are less profitable because almost all gambler loss his btc at casino .