Replace receiver when there is an existing one. (#2611)

The receiver should not change, but code wise, the option of replacing
receiver object makes more sense, i.e. otherwise, it could look like we
are leaving the stale object in there without replacing with new
receiver of same type.

pkg/rtc/mediatrackreceiver.go

@@ -161,19 +161,21 @@ func (t *MediaTrackReceiver) SetupReceiver(receiver sfu.TrackReceiver, priority
 	receivers := slices.Clone(t.receivers)
 
 	// codec position maybe taken by DummyReceiver, check and upgrade to WebRTCReceiver
-	var existingReceiver bool
-	for _, r := range receivers {
+	idx := -1
+	for i, r := range receivers {
 		if strings.EqualFold(r.Codec().MimeType, receiver.Codec().MimeType) {
-			existingReceiver = true
-			if d, ok := r.TrackReceiver.(*DummyReceiver); ok {
-				d.Upgrade(receiver)
-			}
+			idx = i
 			break
 		}
 	}
-	if !existingReceiver {
-		receivers = append(receivers, &simulcastReceiver{TrackReceiver: receiver, priority: priority})
+	if idx != -1 {
+		if d, ok := receivers[idx].TrackReceiver.(*DummyReceiver); ok {
+			d.Upgrade(receiver)
+		}
+		// replace receiver
+		receivers = slices.Delete(receivers, idx, idx+1)
 	}
+	receivers = append(receivers, &simulcastReceiver{TrackReceiver: receiver, priority: priority})
 
 	sort.Slice(receivers, func(i, j int) bool {
 		return receivers[i].Priority() < receivers[j].Priority()