How ClickHouse Deduplicates Retried Inserts with a Content Hash
ClickHouse hashes each inserted block into a UInt128 dedup token and keeps a bounded window of recent tokens to detect retries.
Replication
How ClickHouse Replicates MergeTree: A ZooKeeper Log and a Hash Deduplication
Replication is a ZooKeeper log plus a content-hash deduplication check that rides along with the block-number allocation.