Currently cortex_ingester_chunks_stored_total reports all chunks that were sent to the store; below that level the store may avoid the write and return success, but this isn't visible.
Similarly cortex_ingester_chunk_stored_bytes_total
(Note these metrics were renamed in April 2020)
(Note that even when the Cortex code doesn't detect duplication the underlying store may do so; there isn't much we can do about that in real-time)