Didn't some actress call herself Tuesday rather than
Susan? Similar sounds, and a more familiar word. Or, I could see a particular kind of family naming the first daughter
Penelope and then the second (anything from
Amy to
Zoe) might become Tuppence as a natural progression.
More realistically, the parents of Biggles and
Posie have proved their affection for nicknames used as full names ... perhaps little Biggles was lucky to escape with a mere literary lnfn!