fix(TPC): ignore "ontrackadded" for existing track

Until M69 Chrome used to consistently emit "onstreamadded" when
audio/video stream is added for the first time and then emit track
events only if the stream is modified afterwards (video track replaced).

However it looks like now it can first emit "onstreamadded" and then
additional "ontrackadded" for the MediaStreamTrack that was already
included in the MediaStream signalled by the "onstremadded" event. I
have not managed to figure out what is causing this and the only
difference in the SDP is the fact that the remote peer (JVB) includes
IPv6 candidates. Anyway it doesn't hurt to have such a safeguard.

modules/RTC/TraceablePeerConnection.js

@@ -728,11 +728,6 @@ TraceablePeerConnection.prototype._createRemoteTrack = function(
         videoType,
         ssrc,
         muted) {
-    const remoteTrack
-        = new JitsiRemoteTrack(
-            this.rtc, this.rtc.conference,
-            ownerEndpointId,
-            stream, track, mediaType, videoType, ssrc, muted, this.isP2P);
     let remoteTracksMap = this.remoteTracks.get(ownerEndpointId);
 
     if (!remoteTracksMap) {
@@ -740,11 +735,35 @@ TraceablePeerConnection.prototype._createRemoteTrack = function(
         this.remoteTracks.set(ownerEndpointId, remoteTracksMap);
     }
 
-    if (remoteTracksMap.has(mediaType)) {
+    const existingTrack = remoteTracksMap.get(mediaType);
+
+    if (existingTrack && existingTrack.getTrack() === track) {
+        // Ignore duplicated event which can originate either from
+        // 'onStreamAdded' or 'onTrackAdded'.
+        logger.info(
+            `${this} ignored duplicated remote track added event for: `
+                + `${ownerEndpointId}, ${mediaType}`);
+
+        return;
+    } else if (existingTrack) {
         logger.error(
-            `${this} overwriting remote track! ${remoteTrack}`,
-            ownerEndpointId, mediaType);
+            `${this} overwriting remote track for`
+                + `${ownerEndpointId} ${mediaType}`);
     }
+
+    const remoteTrack
+        = new JitsiRemoteTrack(
+                this.rtc,
+                this.rtc.conference,
+                ownerEndpointId,
+                stream,
+                track,
+                mediaType,
+                videoType,
+                ssrc,
+                muted,
+                this.isP2P);
+
     remoteTracksMap.set(mediaType, remoteTrack);
 
     this.eventEmitter.emit(RTCEvents.REMOTE_TRACK_ADDED, remoteTrack);