" Serious students will know (and if you do not, it is my sadness to inform you) that one of the Petronellas died in the line of duty a few years later. No one has ever known which of them perished, and Petronella herselves is quite firm about never telling anyone. I say ‘herselves’ because another Earth resident Zygon took the fallen Petronella’s place, so that there could be two of them again. All that matters is this: Osgood lives—and so long as the fangirls stand guard on the gates of humanity, so will we. "

― Steven Moffat , Doctor Who: The Day of the Doctor