In Paul's duel with Feyd-Rautha, he is worried when Feyd-Rautha attempts to cheat with a hidden poison needle. But isn't Paul immune to poison? Furthermore, why didn't the snoopers detect the poison?
The snoopers seem to have missed the poison on the blade because it wasn't deadly (the snoopers are evidently only looking for fatal poisons):
He realigned his own metabolism to match this threat and change the molecules of the soporific, but he felt a thrill of doubt. They’d been prepared with soporific on a blade. A soporific. Nothing to alert a poison snooper, but strong enough to slow the muscles it touched. His enemies had their own plans within plans, their own stacked treacheries.
He's then worried about the dart, not because the poison might kill him but because if they've already used one novel poison against him (albeit ineffectively), the next one may have more of an effect.
Set against the backdrop of a knife fight where every split-second counts, you can see why he'd want to avoid being drugged even if he's reasonably confident of being able to overcome its effects:
“You see it there on my hip?” Feyd-Rautha whispered. “Your death, fool.” And he began twisting himself around, forcing the poisoned needle closer and closer. “It’ll stop your muscles and my knife will finish you. There’ll be never a trace left to detect!”