“Duplication” means that the exact same content appears more than once in full. For example, a sentence might be repeated entirely in two different places.
“Overlap,” on the other hand, means that two parts share only a portion of content. In chunking, this typically refers to the end of one chunk being partially included at the beginning of the next chunk to preserve context.
So, in the case of “chunk overlap,” the correct term is overlap, not duplication, because only part of the chunk is shared — not the whole thing.