|Tell my wife I'll be home for dinner.|
|Tell my wife I love her.|
The Law of Conservation of Ninjutsu states that each team of ninjas has the same finite amount of "power" of martial arts, therefore a team with less ninjas has more powerful ninjas. That's why when an action hero takes on many foes, it's no sweat. But when facing a single enemy, he'd better watch out.
Repetition in a novel works the same way. When used once it can be very effective. But with each subsequent use it loses impact. When used too much the reader will grow bored, like the Avengers cutting through hordes of robots.
So it's a good idea to limit instances of repetition to maybe once or twice a novel. And it's important you repeat something worth repeating, to draw attention to an important revelation or a character's breaking psyche. Like so:
I walked among the fallen soldiers until I found Bryan. Dead. My son was dead.
Here the repetition is drawing attention to the mother coming to terms with her son's death, or her mental breakdown—whatever the context of the story leads you to believe. Now imagine another repetition was uttered later:
The dish slid out of my hand and broke against the floor. I broke something again.
That last sentence may have some deeper meaning depending on context. Regardless, see how its presence dilutes the impact of the first sentence ever so slightly? Now imagine another repetition appeared in a later chapter:
Diane left that afternoon. Left me again. For the last time.
Now it's beginning to be too much. Notice how the repetitions have the unintended effect of comparing a dead son and a leaving daughter to a broken dish? Any more of these and the reader will begin to think that this character just likes to repeat her thoughts. This is especially bad in third person novels. When multiple characters use repetition, the reader will assume it's just a writing crutch the author likes to use. Because it is.
Since your repetitions are finite, it's important to use them in the right place. Try not to use it like this:
It had been two days since the battle had ended. Two days since the smell of burning flesh had filled the air. Two days since the barbarians had swept through Hampstead. No survivors.
Here the repetition draws attention away from the more important passages. Is it really that important the reader know this happened two days ago, three times over?
It had been two days since the barbarians swept through Hampstead. No survivors. The smell of burning flesh still filled the air.
Repetition is like a quiet exclamation point. Use it sparingly.