The remembered set of sent copies closed a narrow race (two fast copies
with a read older than the first one's landing) but opened worse ones:
a discarded copy blocked re-copying for the session. Drop it; a copy
waiting in attention now counts instead, also after a restart. A copy
whose id would not fit the outbox key is refused rather than given a
plain id that slips past the duplicate check.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>