Two people did "die again." The title isn't referring to just the murder of his wife, Margaret, but the murder of his dreams. Happened twice, once 20 years earlier, and now when he killed his wife, which inadvertently kills Peggy. And Peggy/Margaret did die twice, in the same way. The spirit of her died 20 years earlier when she lost the idealism she shared with her husband and she physically died at the hand of her husband.