I plan to raise a Togekiss with Serene Grace. Togekiss can learn Air Slash which has a 30% chance of making the opponent flinch. I read from Serebii that Serene Grace doubles the chance of the secondary effect occuring, which would make the flinch chance 60%. If I make the Togekiss hold a Razor Fang, which increases flinch chance by 11.7%, will the chance of flinching become 71.7%? Or is Razor Fang's flinching buff also doubled by Serene Grace, making the overall flinch rate 83.4%?
According to Bulbapedia:
Serene Grace boosts the 10% flinch chance due to King's Rock or Razor Fang to 20%.
But as Studoku commented, in generation 5 Razor Fang increased the chance of causing the target Pokémon to flinch when using a move that deals damage and does not already have a chance to flinch.
It's reasonable to think this has not changed in generation 6.