Greptile Summary

This PR fixes a duplicate-message delivery bug in the followup queue by introducing a short-lived, module-level dedupe cache (RECENT_QUEUE_MESSAGE_IDS) that persists message IDs across queue drain cycles. Previously, deduplification only compared against items currently in memory; once a drain completed and the queue was torn down, a redelivered message bypassed the check entirely. The fix is targeted and the regression test covers the exact scenario described in the root-cause analysis.

The peek-before-enqueue / check-after-push pattern is used correctly against the DedupeCache API, and items dropped by applyQueueDropPolicy are intentionally not recorded in the cross-drain cache to remain eligible for redelivery once capacity is available — consistent with existing in-memory deduplication behaviour. The resetRecentQueuedMessageIdDedupe test helper is correctly scoped and guarded.

Confidence Score: 4/5

• Safe to merge — the fix is correct, logically sound, and well-tested. The cross-drain dedupe cache is checked before any other path and is populated only on confirmed successful enqueue.
• The root-cause fix is sound: the cross-drain dedupe cache correctly prevents duplicate messages across drain restarts, and the regression test directly validates the described failure scenario. The implementation correctly uses the DedupeCache API (peek for read, check for write after push), and the behavior regarding dropped items is intentional and consistent with existing deduplication logic.
• No files require special attention

_{Last reviewed commit: b86b4d9}

This PR should be pushed independently of #33455.

#33455 fixes a different issue (duplicate outbound agent), but this bug and PR address the root cause of duplicate incoming messages. It was referenced as fixing #33150, but it's not a real fix for that issue. It might reduce the visible impact, but duplicates would still be delivered to the agent, still burn tokens, and the agent would still need to process them — the outbound dedup just catches duplicate responses.

Landed on 5edcab2 from source commit b86b4d9.

What was done:

• rebased/landed on latest 'main' via temp branch
• kept the queue redelivery dedupe fix and regression test
• added changelog entry with contributor + PR attribution
• ran full gate before commit: 'pnpm lint && pnpm build && pnpm test'

Thanks @rylena for the fix and repro detail.