It also happened when a child had the same name as a parent, as a result of traditional naming patterns. Polly is said to be a diminutive of Mary. However my Elizabeth had a mother and a sister both named Hannah. I could not find Hannah the younger in 1871, until I found her as a visitor in Elizabeth's household, named Polly. The census form was probably filled in by Elizabeth's husband, who had only ever known his sister-in-law as Polly.