fix(desktop): stop a finished reply rendering twice after history catches up

When a turn's reply commits under its own id, the settled local
`assistant-stream-*` row shifts one assistant ordinal earlier, so ordinal
pairing finds nothing at its slot and re-appends it — the same answer twice.

Drop a settled stream row only when the authoritative transcript already
carries that exact text. Keying `isPendingAssistant` on the explicit pending
flag alone would also have fixed this, but it discards the sibling case in
the same report: a reply that finished locally before the gateway committed
it, where the local row is the only copy that exists.

Co-authored-by: Dolverin <59100064+Dolverin@users.noreply.github.com>

apps/desktop/src/app/session/hooks/use-session-actions/utils.test.ts

@@ -945,6 +945,32 @@ describe('preserveLocalPendingTurnMessages', () => {
     expect(chatMessageText(preserved[1])).toBe('streamed body')
   })
 
+  // #70209: history committed the reply under its own id, so the settled local
+  // stream row sits at a later ordinal, pairs with nothing, and gets appended —
+  // the same answer twice.
+  it('does not re-append a settled stream row the authoritative history already carries', () => {
+    const next = [msg('1-user-stored', 'user', 'question'), msg('2-assistant-stored', 'assistant', 'answer')]
+    const settledLocalStream = msg('assistant-stream-runtime-1', 'assistant', 'answer', { pending: false })
+
+    expect(preserveLocalPendingTurnMessages(next, [...next, settledLocalStream])).toBe(next)
+  })
+
+  // The reply finished locally but the gateway had not committed it when the
+  // session was reopened — the local row is the only copy and must survive.
+  it('keeps a settled stream row the authoritative history has not committed', () => {
+    const previous = [
+      msg('1-user', 'user', 'question'),
+      msg('assistant-stream-sess', 'assistant', 'the finished reply', { pending: false })
+    ]
+
+    const next = [msg('1-user', 'user', 'question')]
+
+    expect(preserveLocalPendingTurnMessages(next, previous).map(message => message.id)).toEqual([
+      '1-user',
+      'assistant-stream-sess'
+    ])
+  })
+
   // The whole point of replacing rather than appending: one reply on screen,
   // and the committed history around the live turn untouched.
   it('does not duplicate or rewrite committed history around the live turn', () => {

apps/desktop/src/app/session/hooks/use-session-actions/utils.ts

@@ -507,6 +507,24 @@ export function preserveLocalPendingTurnMessages(
 
     const authoritative = nextByRoleOrdinal.get(`${message.role}:${ordinal}`)
 
+    // A settled stream row (`pending: false` after message.complete) whose reply
+    // the authoritative transcript already carries under its committed id is
+    // stale: ordinal pairing can't see it, because the commit shifted the row
+    // one ordinal earlier, and re-appending it renders the same answer twice
+    // (#70209). Only text-identical rows are dropped — a settled row the backend
+    // has NOT committed yet is the only copy of that reply and must survive.
+    if (
+      isPendingAssistant &&
+      message.pending !== true &&
+      nextMessages.some(
+        candidate =>
+          candidate.role === 'assistant' &&
+          textWithoutReferenceLines(chatMessageText(candidate)) === textWithoutReferenceLines(chatMessageText(message))
+      )
+    ) {
+      continue
+    }
+
     if (authoritative) {
       if (isPendingAssistant) {
         // Keep the local pending row when it is the same reply further along
@@ -522,7 +540,8 @@ export function preserveLocalPendingTurnMessages(
       }
 
       if (
-        textWithoutReferenceLines(chatMessageText(authoritative)) === textWithoutReferenceLines(chatMessageText(message))
+        textWithoutReferenceLines(chatMessageText(authoritative)) ===
+        textWithoutReferenceLines(chatMessageText(message))
       ) {
         continue
       }