Comment by super_ar

Thanks for asking those questions. Duplicates often come from how systems interact with Kafka, not from Kafka itself. For example, if a service retries sending a message after a timeout or if you collect similar data from multiple sources (like CRMs and web apps), you can end up with the same event multiple times. Kafka guarantees delivery at least once, so it doesn't remove duplicates.

ClickHouse doesn't enforce primary keys. It stores whatever you send. ReplacingMergeTree and FINAL are concepts on ClickHouse, but they are not optimal for real-time streams due to the background merging process that needs to be finished to ensure correct query results.

With GlassFlow, you clean the data streams before they hit ClickHouse, ensuring correct query results and less load for ClickHouse.

In your IoT case, a scenario I can imagine is batch replays (you might resend data already ingested). But if you're sure the data is clean and only sent once, you may not need this.