Seems if you eat poisoned food you can get poisoned, twice. Since you cannot cast it on someone who has it already, I figured this was a bug. Here is the log:

<100%hp 100%m 100%mv 1241tnl> eat steak
You eat a steak of the corpse of An eating dwarf servant.
You choke and gag.

<100%hp 100%m 100%mv 1241tnl> g steak sack
You get a steak of the corpse of An eating dwarf servant from a large sack.

<100%hp 100%m 100%mv 1241tnl> eat steak
You eat a steak of the corpse of An eating dwarf servant.
You are full.
You choke and gag.
You can no longer find the strength to wield a practice sword.

<100%hp 100%m 100%mv 1241tnl> aff
You are affected by:
Physical malady: 'poison'.
Physical malady: 'poison'.