How did you prevent duplicate processing?

Short explanation:

Duplicate prevention identifies repeated attempts at the same logical operation and avoids applying its effect twice. It must work under concurrent requests and crashes, so an ordinary pre-check is often insufficient. A stable key plus an atomic database rule is one approach, with separate handling for external effects.

Key points:

• Describe the actual unique key, database constraint or provider idempotency support.

• If absent, explain that an atomic key check plus state change would be an improvement.

• Explain where the operation ID originated and how the stored result or status was reused on a repeated request.

Interview point:

• Do not claim exactly-once behavior unless you can explain the crash window between every important step.