The iocane powder trick with the Vizzini meta-twist

Where does this variation on Mithridatism come from?

Stop reading right now if you want to avoid SPOILERS for various mystery plots!

The other day, I was reading about the plot of the last novel Agatha Christie published before her death—and the last Hercule Poirot novel—Curtain (1975). I haven't read the novel, and I was unaware of this plot twist: Poirot plans to drug the killer using sleeping pills in hot chocolate; the killer anticipates the trick, so he insists on drinking Poirot's hot chocolate; but Poirot anticipated the killer's move and drugged both cups. Poirot had a higher tolerance for the sleeping pill, so the killer, but not Poirot, ended up drugged.

Of course, this is the iocane powder trick. The Princess Bride movie came out in 1987, but this trick was also present in William Goldman's novel, which was published in 1973, two years before Christie's publication of Curtain.

So was Christie copying Goldman? But Christie's novel was written in the 1940s and locked away in a bank vault for decades. Perhaps this plot twist was already present in the original? But did Goldman know about the plot of Curtain? Did he know Christie? Were the plot details known to Christie's friends? Both books have what I'll call the Vizzini twist, where the target has the idea of switching cups. But the implementation in The Princess Bride has the Vizzini meta-twist, where Vizzini follows the logical reasoning to an absurd degree—which also supports the idea that Goldman knew of a version with a single twist.

Now the basic idea has a long history: Mithridatism is the idea of protecting yourself against poison by taking small doses (see, e.g., Terence, or The Count of Monte Cristo). The more involved idea, where you use your immunity to kill someone else (and, at the very least, distract attention from yourself), was also used a lot before 1973, for instance in Dorothy Sayers's Strong Poison, and Dashiell Hammett used similar ideas, as did Christie herself in some of her other novels (see, e.g., Peril at End House).

But I haven't been able to find any pre-1973 cases (aside from the unpublished Curtain) that had not only the iocane powder trick but also the Vizzini twist. Does anyone know of any, or have any insight into whether Goldman knew about Curtain?