"One of the general patterns of good (i.e., striking and memorable) writing is the effect of repetition. If you use a certain element-a plot device, an image, a noticeable phrase-once, readers may or may not notice it consciously, but it doesn't disturb the flow of their reading. If you use that element twice, they won't notice it consciously-but they will notice it subconsciously, and it will add to the resonance of the writing or to their sense of depth and involvement (and if it's a plot device, it will heighten the dramatic tension). But if you use that element three times, everybody will notice it the third time you do it."