fix(parent-wake-notifier): drop duplicate wakes during promptAsync gate hold (#4256, #4019)

In v4.1.0+, users observed duplicate assistant streams rendering the same
content in two languages simultaneously (e.g. Chinese + English), most often
at the end of a turn.

Root cause: ParentWakeNotifier.requeueWake() unconditionally requeued ANY
wake that arrived during the background-agent-parent-wake post-dispatch
hold window. When a duplicate completion edge fired during that hold, the
same wake was replayed after the hold expired, triggering a second prompt
dispatch and a parallel assistant stream.

The fix compares the new wake against dispatchedParentWakes.get(sessionID)
and drops identical wakes during the gate hold, while preserving the existing
requeue behavior for genuinely-new wakes and failed-dispatch retries.

Regression test added in given/when/then style covering the duplicate-during-
hold scenario (TDD red-then-green).

Fixes #4256
Fixes #4019

src/features/background-agent/parent-wake-notifier.ts

@@ -238,6 +238,13 @@ export class ParentWakeNotifier {
         throw promptResult.error
       }
       if (promptResult.status === "reserved" && promptResult.reservedBy === "background-agent-parent-wake") {
+        const dispatchedWake = this.dispatchedParentWakes.get(sessionID)
+        if (dispatchedWake && this.isSameParentWake(latestWake, dispatchedWake)) {
+          // #4256/#4019: duplicated completion edges can enqueue the same wake
+          // during the gate hold. Replaying it later starts a second assistant stream.
+          log("[background-agent] Suppressed duplicate parent wake during promptAsync gate hold:", { sessionID })
+          return
+        }
         this.requeueWake(sessionID, latestWake)
         this.schedulePendingParentWakeFlush(sessionID, 2_000)
         log("[background-agent] Requeued parent wake flush reserved by promptAsync gate hold:", { sessionID })
@@ -399,6 +406,12 @@ export class ParentWakeNotifier {
     this.dispatchedParentWakeTimers.set(sessionID, timer)
   }
 
+  private isSameParentWake(left: PendingParentWake, right: PendingParentWake): boolean {
+    return left.shouldReply === right.shouldReply
+      && JSON.stringify(left.notifications) === JSON.stringify(right.notifications)
+      && JSON.stringify(left.promptContext) === JSON.stringify(right.promptContext)
+  }
+
   private async loadParentWakeSessionMessages(sessionID: string): Promise<ParentWakeSessionMessage[]> {
     try {
       const messagesResp = await messagesInDirectory(this.deps.client, {

src/features/background-agent/parent-wake-same-source-requeue.test.ts

@@ -84,6 +84,31 @@ function releaseParentWakeHold(sessionID: string): void {
 }
 
 describe("ParentWakeNotifier — same-source reservation requeue (BUG-E)", () => {
+  test("#given a duplicate parent wake is in post-dispatch hold #when the duplicate fires again #then it is dropped instead of requeued", async () => {
+    // given
+    const { notifier, promptAsyncCalls } = createNotifier()
+    const sessionID = "parent-hold-duplicate-wake"
+    notifier.queuePendingParentWake(sessionID, "wake A", { agent: "sisyphus" }, true)
+
+    try {
+      await notifier.flushPendingParentWake(sessionID)
+      expect(promptAsyncCalls).toHaveLength(1)
+
+      // when
+      notifier.queuePendingParentWake(sessionID, "wake A", { agent: "sisyphus" }, true)
+      await notifier.flushPendingParentWake(sessionID)
+      releaseParentWakeHold(sessionID)
+      await notifier.flushPendingParentWake(sessionID)
+
+      // then
+      expect(promptAsyncCalls).toHaveLength(1)
+      expect(notifier.getPendingParentWakes().has(sessionID)).toBe(false)
+    } finally {
+      notifier.shutdown()
+      releaseAllPromptAsyncReservationsForTesting()
+    }
+  })
+
   test("#given a parent wake is in post-dispatch hold #when a new pending wake fires within the hold window #then the new wake is re-enqueued and dispatched after the hold expires", async () => {
     // given
     const { notifier, promptAsyncCalls } = createNotifier()