Postman rings twice

[Douglas G. Wilson]

Does anybody here know the origin and significance of the saying "The
postman [always] rings twice"?

This seems to be some sort of aphorism or metaphor. It was the title of a
famous novel, which I read long ago ... I don't remember a postman in the
novel, but ....

I looked through some reference books (books of proverbs, sayings, etc., at
a small library) without any luck. Can somebody suggest a reference?

-- Doug Wilson