Why do queue consumers need to be idempotent, and how would you design that?

Design for at-least-once delivery with idempotent handlers

Explains why message handlers must tolerate duplicate delivery and how idempotency protects side effects.

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TL;DR

Most practical queues give at-least-once delivery, which means a message can be delivered more than once. That can happen after worker crashes, lock expiration, network failure, or uncertain acknowledgement. I would design the handler so processing the same message twice does not create duplicate business effects. That usually means a stable message id, business idempotency key, or unique database constraint.

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At-least-once delivery, retries, duplicate messages, idempotency keys, database constraints, and safe side effects.

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Why this question matters
Duplicate delivery is normal in an at-least-once system because work and acknowledgement cross separate failure boundaries. Handlers must make repeated delivery safe for every durable and external side effect.
Most practical queues give at-least-once delivery, which means a message can be delivered more than once. That can happen after worker crashes, lock expiration, network failure, or uncertain acknowledgement. I would design the handler so processing the same message twice does not create duplicate business effects. That usually means a stable message id, business idempotency key, or unique database constraint.
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I would make idempotency part of the business operation, not just a cache entry. For example, if a message creates an invoice, the invoice can have a unique key from tenant id plus source event id. If the handler sees the same key again, it returns the existing result or skips the already-applied change. External side effects like email or payment calls need their own idempotency keys or an outbox/inbox boundary.

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Assuming a message is processed exactly once.
Using an in-memory duplicate check that disappears on restart.
Making external side effects before recording idempotency.
Not keeping idempotency records long enough.

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Why do queue consumers need to be idempotent, and how would you design that?

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