To double the number of +1/+1 counters on a permanent, put a number of +1/+1 counters on it equal to the number it already has. Other cards that interact with putting counters on it will interact with this effect accordingly.

If you put Fractal Harness onto the battlefield without casting it, or if you choose 0 as the value of X, the token you create will be 0/0. Unless something else is raising its toughness, that token will enter the battlefield and die before you could take any actions.