Rollups improve blockchain scalability by aggregating transactions, yet sequencer centralization introduces risks like ordering manipulation and censorship, which are difficult to verify. While current approaches focus on distributed sequencing or state correctness, they overlook the need for ordering accountability. We introduce DOPPIO, a framework for retrospective verification of fair sequencing. DOPPIO employs a dual-commitment scheme, binding the sequencer to both the ordering metadata and execution batch, and extends the base detector with value-weighted and spatial-concentration variants that, aggregated over consecutive batches, detect sustained localized maximal extractable value (MEV) attacks. We characterize the per-pair limit of observer-based detection: sub-margin delay masking of an isolated transaction pair is indistinguishable from network jitter. Validated on real Ethereum mainnet mempool data, DOPPIO provides accountability with negligible communication overhead and no added execution latency.
