<![CDATA[At-least-once delivery is the dominant message-delivery paradigm in distributed messaging infrastructures, because stronger transport-level guarantees require blocking coordination that compromises availability and throughput. This paper gathers, from the foundational and modern literature, the concepts of message delivery semantics, of idempotency theory and of distributed coordination patterns, articulating a conceptual framework, called semantic idempotency, where exactly-once business effects are achieved on top of an at-least-once transport, by relying on deduplication performed on the receiver side instead of on the transmitter side. It starts by tracing the history of the various studies that led to the development of logical clocks and consensus impossibility theorems, quorum-based replication, log-structured messaging, snapshot algorithms, saga-based compensation, and conflict-free replicated data types, and how each addresses a partial solution to the duplicate-delivery problem. The analysis reveals that redelivery probability increases non-linearly with the number of retries at realistic acknowledgment-loss rates, the idempotency-key registry adds approximately one millisecond of median latency compared with mid-single-digit milliseconds for two-phase commit, and windowed key-store retention can maintain over 95 percent accuracy of duplicate-catching while keeping state growth under control. Idempotent receivers preserve the availability and horizontal scalability of at-least-once transport, at the cost of weaker native consistency guarantees than two-phase commit provides. The paper also presents the formalization of semantic idempotency using an operator-oriented notation that separates redelivery at the transport level from application of effects at the business level. These results suggest that architects should regard idempotency as a first-class contract between product and consumer, while the future of messaging systems might be a platform that includes a deduplication registry as a first-class citizen rather than an ad hoc application programming feature.]]>
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