Three Core Conditions That Guarantee Deduplication in Async Systems

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A software developer has shared a reasoning-first framework for understanding duplicate processing in asynchronous message flows, focusing on root causes rather than jumping to technical solutions like Redis or the Inbox pattern. The core insight is that duplicate processing is not about how many times a message is delivered, but how many times its side effect is actually applied. According to the analysis, duplication becomes harmful only when the repeated side effect is not idempotent, meaning it produces unintended cumulative results. The author identifies three failure conditions that cause duplicates: missing unique identifiers, non-atomic writes of the side effect and its evidence, and evidence stored only in a single consumer's local memory. Flipping these failures yields three guarantees — a unique identifier per intent, atomic evidence recording, and globally visible proof of execution — which together form the foundation for reliable deduplication.

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