What is it and why is it bad?
It's boring to read the same thing over and over. And it's really annoying when it was several pages ago and more than once. Macro-repetition is a far more serious problem. It's bigger. It's knottier. It means you either didn't have a clear goal for the scene containing the repetition or you got sidetracked. It's easier to fix because you delete it.
Repetition on the macro-level.
Macro-repetition frequently occurs when backstory gets repeated. The hero, for example, tells someone he hates dinner parties. Three pages later, the hero's mother mentions to someone else the hero hates dinner parties. A few pages later, the hero's best friend mentions the hero hates dinner parties.
Delete everything that doesn't serve a unique function and advance the story. The section has to do both. At the same time. If you can delete the passage without really affecting the story, delete it.
Enough all ready. We know the hero hates dinner parties. Quit telling everyone the same dang thing!
The solution is. . . Backstory!
Just friggen tell us the hero hates dinner parties. Don't out-clever yourself by trying to figure out how to show us. Just tell us. Once.
But how do you know when it's happening, again?
Because your characters are standing around telling each other something we all ready know. Just because they're in a different room or new people are there doesn't give you an excuse to tell us something again. The story isn't going anywhere.
See, you leave it in if the information being re-presented creates some tension or is the hook on which you're hanging an important plot point. (Well, in which case, why didn't you just do that the first time?) OK, so you leave it in the second time only if the re-presentation is itself the hook. Otherwise, you should have found a way to do this the first time you brought it up.
Another kind of Macro-Repetition
It's not just backstory. Entire scenes or chapters can be macro-repetitive. How depressing. And how true. It's hard to give an example without boring you to death. You just have to make sure each scene and or chapter serves a unique and necessary function within the novel.
Read Chapters 1 through 5 of your WIP. Delete the stuff you already said before.
Read every chapter of your WIP. Delete everything that doesn't serve a unique function and advance the story. The section has to do both. At the same time. If you can delete the passage without really affecting the story, delete it.