Reviews the code that checks a value and then acts on it: balances, stock, coupons, one-time tokens, and quotas. Paste the handler, the queries, the transaction setup, and the schema constraints to trace how two concurrent or repeated requests can spend, redeem, or claim the same thing twice. Returns a verdict, an interleaving map, evidence-linked findings, the smallest safe fix, and a concurrent test that proves the limit holds. Written for backend developers who ship payment, credit, and redemption endpoints. States the isolation level and constraints it does not see instead of inventing them. A code review prompt, not a load tester or a formal proof.