Is implementing idempotent event processing an achievement for a senior, or is it something that is expected of them a priori?

Implementing idempotent event processing is more of a basic requirement for a senior than an achievement. Idempotency ensures that processing the same event repeatedly will not lead to errors or data duplication, which is critical in distributed systems.

A senior is expected not only to understand this principle but also to be able to implement it correctly, considering the specifics of architecture and business logic. That is, idempotency is a quality standard, not an additional bonus.

If a candidate demonstrates a deep understanding, can design complex idempotency mechanisms, and optimize them, this is already a plus and a sign of a mature specialist.