Apache Pinot

Does it support any exactly once guarantees like Druid ?

Are you referring to: <https://imply.io/post/exactly-once-streaming-ingestion>

If so, it is unclear to me if exactly once implies de-duping duplicate Kafka messages (even without failures). Or does it mean avoiding duplicates when recovering from failure? If latter, Pinot has always had that. If former, Pinot does not de-dup duplicate events during ingestion, atm.

I mean does it process every "kafka offset id" only once ?

It may reprocess to recover from failure, however, the every incoming event is indexed and stored only once (your queries won't have duplicate events because of Pinot).