ToME WikiRobinMunn: (2.3.3) I acquired The Elfstone 'Elessar' on top of Mt. Doom, as a drop from a unique (specifically, Gothmog, the High Captain of Balrogs). Now I'm diving through Angband, and a Great Wyrm of Law just dropped... the Elfstone 'Elessar'.
I'm guessing this has something to do with Mt. Doom's persistent nature -- and so BugReport334 (companion duplication) may also be caused by the same thing.