@ToKittyornottoKitty , Molly, Polly and May are forms of Mary.
In times past it was common for grandchildren to be named after grandparents, so you'd have cousins both called Anne but one would be Annie and the other one Nancy etc.
Henry known as Harry has been around for centuries - “Cry ‘God for Harry, England, and Saint George!’” – William Shakespeare, Henry V, Act III, Scene I.