“The horror! The horror!”
— Joseph Conrad, Heart of Darkness (1899)
Context
Kurtz, the ivory trader who descended into atrocity in the Belgian Congo, dies whispering these words on a steamer headed downriver. Marlow, the narrator, hears them and never resolves whether Kurtz meant the horror of his own deeds, the horror of human nature, or both.
How the repetition works
Two identical exclamations, no elaboration. The ambiguity is the point: what "horror" refers to is precisely what Conrad refuses to fix. The repetition forces the reader to hold the word twice without resolving its content, the way Kurtz himself could no longer resolve what he had seen and done.