Question about Supernatural episode 'Jus in Bello'?

Everywhere I look, it says that in that episode of supernatural, Aimee Garcia plays secretary Nancy Fitzgerald (I know that - I remember her), but then it says that Aubrey Plaza plays Nancy Fitzgerald. And that just confuses me, because she so was not in Supernatural, and why on earth would 2 actresses be playing the same character anyway? So anyway, I was just wondering if anyone could clear up the reason behind this weirdness?