Two ledger flushes that upsert the same new day (or touch the same new instance) at once made one of them fail on the unique key, losing its counts; MongoDB retries that only for single-document updates. The loser now tries again and adds to the document the winner made. Seen as a rare failure of LedgerOverHttpTests in full runs. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com> Claude-Session: https://claude.ai/code/session_01LsXgEaXee4GCU1hwYgPJXw