“I undid the lantern cautiously — oh, so cautiously — cautiously.”
— Edgar Allan Poe, The Tell-Tale Heart (1843)
Context
The narrator is describing how, on the seventh night, he crept into the old man's room intending to commit the murder. The whole story is told as evidence of the narrator's sanity, and his fussy attention to method is meant to prove his rationality — but actually demonstrates the opposite.
How the repetition works
The repetition of "cautiously" performs the action it names: the line itself proceeds slowly, with interruption, qualified by "oh, so cautiously" before circling back. The effect is unsettling — what the narrator means as proof of careful planning sounds, to the reader, like the obsessive looping of a deranged mind.