On-callMediumoc-g342

Subject Duplicate processingLevel Mid–Senior~35 minCommon in Networking & APIs interviewsIndustries Technology

Question

Your order API is idempotent: clients send an `Idempotency-Key` header and the server dedupes by it. After a deploy of the mobile app at 09:00, support reports a rising trickle of *duplicate orders* — same cart, two orders seconds apart — only from mobile, not web. Dashboards: API error rate flat, p99 normal, the idempotency table is filling faster than usual. The mobile change added an automatic retry on network timeout. How do you triage, stop the duplicates, and reconcile?

What a strong answer looks like

Stop the bleeding first (mitigate), then form hypotheses from real signals. Separate root cause from symptom, communicate status as you go, and close with what prevents a repeat.

Learn the concepts

Diagram & narrate the incident

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Run or narrate your approach, then ask the coach.