On-callMediumoc-g352

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

Question

You receive payment webhooks from a third-party provider. Your handler records the payment and ships the order. The provider retries any webhook it doesn't get a 2xx for within 10s. Starting at 12:00, a slow downstream made your handler take 12–15s; afterward, ops finds duplicate shipments and duplicate ledger rows for a subset of payments. Dashboards: handler error rate flat (it returns 200, just slowly), order/shipment volume elevated. How do you triage, stop the duplicate side-effects, 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.