We all have comfortable patterns in our writing that we fall back on, like taking the same route to work every day. Just as people eat comfort food when distressed, writers rely on familiar sentence constructions when a passage is hard to write or emotionally charged.
As an editor, I see four common examples of repetition. Rather than pulling examples from work I've edited, I decided instead to create them while watching my daughter's tae kwon do class.
The first example is writing sentences of the same length, as in these instructions: "Both hips should be square. Tap your leg and step. Use your left arm next." Establishing a rhythm can be lyrical, but never varying it is boring.
The second example of repetition strings together two independent clauses, joined by "and" or "but." "Today the students reviewed the 12 basic stances, and more than 20 children attended. The students practiced each pose, and the instructor gave them tips. Most of the children tried very hard, but a few of them were distracted."
Appropriately, the third example deals with "threes," series of them. Writers may include three independent clauses in a sentence or three items in a list. This passage contains both: "The class time is separated into equal segments for warming up, cardiac work, and poses. First, the children stretched out, then they ran laps, and next they practiced poses. The students demonstrated the low back stance, the low twist stance, and the front stance."
The last example of repetitious construction is putting dependent clauses in the same place for every sentence. In this example, all the clauses come at the beginning of the sentences: "Before children step onto the mat, they must bow. When class begins, they form a line. After the initial greeting, the children do stretches to warm up."
Examine your writing at the paragraph level. If you have two or three sentences with the same construction in one paragraph, then it's time to shake things up. Look for ways to recast the sentence. Like moving furniture in a room, sometimes shifting things around can give you a whole new look.