It can be:
* Repetitive and boring: A sentence like "The cat sat on the mat, the cat sat on the mat" is likely to be dull.
* Emphasizing a point: If you use repetition strategically, it can draw attention to a specific word or idea. For example, "No, no, no! I will not go!" emphasizes the refusal.
* Creating a rhythm: Repetition can create a sense of rhythm and flow, especially in poetry or songs. Think of nursery rhymes like "Humpty Dumpty sat on a wall, Humpty Dumpty had a great fall..."
* Building suspense or tension: Repeating a word can create a sense of anticipation or dread, especially when combined with other literary devices.
* Illustrating a mental state: Repeating a word could show confusion, obsession, or madness in a character.
Examples:
* Emphasize: "Stop, stop, stop! I said stop!"
* Rhythm: "The rain falls on the roof, the rain falls on the street, the rain falls on the grass, the rain falls on the feet."
* Suspense: "Tick, tick, tick, tick..."
* Madness: "No, no, no. He's gone, he's gone, he's gone..."
Ultimately, the effect of a sentence with repeated words depends on the context and the author's intention.