Backend engineer

Refunds are where payments go wrong: the same request arrives twice because a network retried it, and two refunds is an incident. I made every step idempotent and tested the duplicate arrival as the normal path rather than the exception.

On reconciliation, the daily manual comparison against the processor's statement became an hourly automatic one that only surfaced differences. Those went from dozens a day to one or two a week, each traceable to a specific request.