As I understand it, Duergar are meant to be immune to all forms of poisons. My Duergar has rolled and failed saves vs. poison trap, poison arrows and poison weapons however. I think immunity: poison needs to be added to the grey dwarf skin.

I thought they have hardiness vs poisons? Not immunity?

Monster Manual (3.5) lists them as having immunity to poison, and that said immunity replaces the Hill Dwarves +2 save bonus toward poison (obviously). Haven't got the Faerûn Campaign Setting, so can't doublecheck it.

Immunity to magical poisons only, not alchemical poisons. So you are immune to the spell Poison, but not other poisons.