Why the Postman Always Rings Twice

I found this note about knocking customs in 19th-century England on page 60 of Clara Kathleen Rogers's aforementioned memoir (which I've finished). Young Clara is discussing one flight of imagination that led her to believe that there was actually a person of importance knocking at the door. Here's her footnote about door knockers:

This was the period of iron and brass knockers, the street door of every house being furnished with one. Even these knockers were made to register "class distinctions." For instance, there was one knock for those who came on business of a humble nature, two for the postman, three or four for professional visitors, while for social calls from the upper classes from five to eight knocks were customary; this was called by the name of "a double knock" and was often followed by a lusty peal of the door-bell, which generally indicated that there was a carriage and a pair at the door.

Perhaps the reason for the title of the movie (and the novel from whence it came) has something to do with knowing who it is knocking on the door, or what is simply bound to happen.