Background/subagent task notifications injected twice per completion ([queued] + delivered), flooding orchestration sessions

In agent-heavy sessions, every background-task / subagent completion notification appears twice in the conversation: once as a [queued] message and once when delivered. Each copy is a multi-KB payload (the full result block), so orchestration sessions end up with the large majority of their "user messages" being duplicated notification blocks.

Observed impact (from local transcript analysis, versions 2.1.152 → 2.1.205, Linux):

• One session: 34 of 39 user-role messages were notification blocks, nearly all duplicated pairs.
• Another: 52 queued messages, almost all duplicates of already-delivered notifications.
• Another: 70 queued entries for a much smaller number of actual completions.
• Net effect: tens of KB of redundant context per orchestration session; real user instructions get buried, and terse follow-ups ("do all of them") become referent-ambiguous for the model.

Expected: one notification per completion event (or dedup of the queued copy once it has been delivered).

Environment: Linux; reproduced across CC 2.1.152 → 2.1.205.

Happy to provide scrubbed transcript excerpts on request.

Related (not duplicates — different mechanisms): #47930 (Agent Teams task_assignment echo dup), #66219 (plugin monitor re-delivery per turn), #74637 (queued-message desync with background agents), #20754 (multi-agent completion notifications not delivered).