A queued message is removed from the queue only after its send resolves,
so between dispatch and resolution it stays visible to every reader — and
a composer submit merges the whole queue into its own send. Over a relay
that window is seconds, long enough to deliver the same message twice.
The queue now tracks which entries are awaiting the server. Dispatchers
skip them, clearQueue retains them so the pending send can still remove
or restore its own entry, and the flag is not persisted because a restart
has no in-flight sends.
Bohdan Triapitsyn
237cae16b3