...

Note: an "update" is a concept that only applies to Table operations, so the concept of an "idempotent update" also only applies to Table operations.
See https://kafka.apache.org/documentation/streams/developer-guide/dsl-api.html#streams_concepts_ktable for more information.

Given that definition, we propose for Streams to drop idempotent updates in any situation where it's possible and convenient to do so. For example, any time we already have both the prior and new results serialized, we may compare them, and drop the update if it is idempotent.

Note that under this proposal, we can implement idempotence checking in the following situations:
1. Any aggregation (for example, KGroupedStream, KGroupedTable, TimeWindowedKStream, and SessionWindowedKStream operations): 

Metrics

The following metric will be added to record the number of idempotent updates dropped:

...