Cross-session message duplication: same user message (identical UUID+ms timestamp) delivered to multiple parallel sessions; duplicate JSONL log lines

Environment: Claude Code on Windows 11, multiple (~6) concurrent sessions in the same project directory.

Symptom 1 — cross-session delivery: A user message typed in session A was also recorded in session B. Both JSONL entries have an IDENTICAL message uuid and IDENTICAL millisecond timestamp, but different parentUuid (each attached to its own session's prior message). The assistant in session B (unintended recipient) acted on it, causing incorrect course-reversal in unrelated work.

Symptom 2 — duplicate log lines: In the source session's ~/.claude/projects/<proj>/<sessionId>.jsonl, the same entry (identical uuid + timestamp) is written 4 times consecutively.

Impact: A misrouted instruction caused the assistant to take wrong actions in the wrong project context.

Evidence: two session JSONL files contain the same message uuid; one session shows 4x duplicated consecutive lines. Repro steps unknown; correlated with many concurrent sessions sharing one working tree.

Ask: Is message routing keyed correctly per-session? Possible race in the log writer duplicating/broadcasting entries across sessions.