There’s something called the Law of Diminishing Returns and I apply it a lot to fiction when I give notes. It has several different applications, but the point behind each is the same: every time something is repeated, it has to be different.
Take, for example, action sequences in a novel or film. They sure are exciting. Until you have five of them in a row and they start feeling boring. That’s the Law of Diminishing Returns in action. Or sex scenes in a romance novel. Or conversations between friends that are meant to be funny. These can all have impact on a reader or viewer, but you have to be very careful with any repeating elements in your story.
The golden ideal in fiction is to have your action, relationships, imagery, tension, stakes–everything–build as you near the climax of your story. Your plot cannot plateau, and it certainly can’t slow down, as you go. Everything must also grow in significance. But if you have some redundant elements, like lots of classroom scenes or several fights between your protagonist and antagonist, those will lose significance and power each time and threaten to drag your plot down.
When you’re doing revision, go through your manuscript and isolate everything that repeats, whether it’s an encounter between characters, a setting, or a plot point. Then make sure that each is different enough from its predecessors and also that you craft its impact slightly differently from all the other times. If it’s a fight with a couple, let this fight plant a seed of doubt in the character’s mind about the future of the relationship. Let the next one inspire the character to stick it out and work through the issue. Let the final fight lead to a bout of the silent treatment, or whatever.
Sometimes you have to have things happen multiple times in a plot. If you can’t change that, change the impact or the significance or the character’s takeaway.
There are lots of ways to manage this issue and keep readers from experiencing Diminishing Returns. Being aware of the problem is the most important step toward fighting it.