When you gamble, you're also able to double your money. For example playing the roulette: bet black, you're able to double your money (but there's 19/36 chance that you won't). I don't think it's a clear scam, but I admit, semantics make it borderline.
When you bet black in roulette there is
a chance that you will be able to double your money. It's by no means certain. There is a chance that you will not able able to double your money too.