Any website or service that promises to 'double your coins' is almost certainly a scam. Do not send them any bitcoin, or any other form of money.

Think about it this way:

• Any sort of trading is a zero-sum game, there is not an infinite amount of BTC out there, to continuously double everyone's coins to infinity. Any trading strategy / whatever this website claims to do in order to double your coins, will involve a counterparty to the trade. Where else would they get the bitcoins from?
• With that in mind, if you were the website operator, and you actually had a fool-proof method to double up bitcoin, would you use it to double your money, or offer your service to others, to double their money? Your method cannot work infinitely, since it is a zero-sum game. So why let someone else take advantage of your method, instead of using it yourself?

The answer to this should be obvious: there is no 'foolproof way to double bitcoins'. Anyone claiming to have one is just trying to scam you.