If multiple replacement effects could apply to damage being dealt to you or Goldnight Castigator, you choose the order to apply those effects.

If you control two Goldnight Castigators and a source would deal damage to you, it deals four times that much damage instead. If you control a third Goldnight Castigator, the source deals eight times that much damage to you, and so on. Damage dealt to a Goldnight Castigator is only doubled, regardless of how many are on the battlefield.