That's interesting because that crops up in the Mabinogion (a collection of Welsh medieval legends) too. Pwyll, the Prince of Dyfed swaps places with Arawn, the Lord of Annwn (the Otherworld) and fights a duel for him. His opponent is Arawn's enemy Hafgan, who can only be killed with a single blow. As he lies mortally wounded following the combat, Hafgan begs Pwyll to finish him off with another blow, but Pwyll has been warned that a second blow will restore Hafgan to health. He doesn't fall for it and Hafgan dies. I wonder if there's some sort of tie-in there with Hafgan being a magical otherworldly creature and all.