Finding Mary Head

In my post, The Two Minnies, I wrote about finding two girls in the Charles Head family named "Minnie", either of whom might be my grandmother, Mary Head. After some research, I settled on the younger of the two, born around 1875. But that left unresolved the more fundamental question of why there were two girls in the family with the same name (or at least nickname) and where that second girl came from.