It denotes something extra amount of WTFness that can not be represented by just WTF. First 'W' is to emphasize the impact of WTFness.
Also it shows that its that big amount of WTFness that its taking a bit of time for digestion.
* mary was mommy of jesus and joseph was her husband and they never had sex
john: dude, do you know how I can calm down a wild bull?
mark: give him a handjob
john: What? What the FUCK? (WWTF)