This is the floor of the entire delivery-semantics sub-domain and it sits directly on the entry floor's single networking assumption ('a call over a network can fail or be slow'). The outcome demands the learner be able to say 'whether a record can be lost or processed twice and what it costs to prevent each'; that sentence is meaningless until they understand that the loss/duplicate fork is forced by the ambiguity of a failed call and is not avoidable by configuration. Every other node in this sub-domain — acknowledgement placement, idempotency, deduplication, the two-writes problem — is a response to this one fact, so nothing above it can be reasoned about correctly without it.
An HTTP request method is idempotent when the intended effect on the server of multiple identical requests is the same as the effect of a single such request; the standard classifies GET, PUT, and DE…