I don't see any way to double your coins by investing in any online site, as that is simply impossible, its better to find and slow and steady way to earn bitcoins instead of going for fast and risky way.
Exactly. The faster the riskier. Thus, the slower the safer.
Everything is in perfect equilibrium. We can't change that nature.
You can double your coins if you double your efforts, and its always good to choose the safer way to earn bitcoins instead of going for risky way where there are more chances of loosing money.