Paul, this is absolutely one of those rules you can forget until you need it.
If a Vampire walks into a bar and spends twenty minutes trying to seduce people, one by one, then this is the rule you are looking for.
Basically, a couple times, players started to slide into a pattern of "playing a single move" rather than "playing their character." They wracked up ten Strings without being interesting or adding depth to the game. The Singleton Rule exists in order to say, "Okay, but you've already gained Strings with that move this scene. So what new thing are you doing?"