When telling bedtime stories, it's crucial to tune in to the desires of the child, in addition to the lessons you feel he or she needs. That is, you want to give the child what they want too. Story time needs to be fun, if it's to be effective.
Yes, you must focus on what the child needs, but you must also keep your eyes open to what the child thinks is fun… the stories he or she wants.
Sometimes a child will want you to repeat the same story day after day. And so long as you don't see an overriding reason not to, this is fine; it may get boring for you, but it's not boring for the child. Remember that the mere pleasure of experiencing a story is, by itself, a potent seed to plant in your child.
One of my friends, a freakishly high IQ guy, is convinced that the pleasure of discovery – the chemical and emotional rush of discovery – is what drives intelligence. I'm not certain he's entirely correct, but I am sure there's substance in his theory, and I think the pleasure a child gets from a good story works in a similar way. So, go ahead and repeat unless you have a reason to do otherwise.