diff --git a/pc/webrtc_sdp.cc b/pc/webrtc_sdp.cc
index ea433583ab..1069a18391 100644
--- a/pc/webrtc_sdp.cc
+++ b/pc/webrtc_sdp.cc
@@ -2341,6 +2341,7 @@ static bool ParseMsidAttribute(absl::string_view line,
   // Note that JSEP stipulates not sending msid-appdata so
   // a=msid:<stream id> <track id>
   // is supported for backward compability reasons only.
+  // RFC 8830 section 2 states that duplicate a=msid:stream track is illegal.
   std::vector<std::string> fields;
   size_t num_fields = rtc::tokenize(line.substr(kLinePrefixLength),
                                     kSdpDelimiterSpaceChar, &fields);
@@ -2670,6 +2671,21 @@ static std::unique_ptr<MediaContentDescription> ParseContentDescription(
   return media_desc;
 }
 
+bool HasDuplicateMsidLines(cricket::SessionDescription* desc) {
+  std::set<std::pair<std::string, std::string>> seen_msids;
+  for (const cricket::ContentInfo& content : desc->contents()) {
+    for (const cricket::StreamParams& stream :
+         content.media_description()->streams()) {
+      auto msid = std::pair(stream.first_stream_id(), stream.id);
+      if (seen_msids.find(msid) != seen_msids.end()) {
+        return true;
+      }
+      seen_msids.insert(std::move(msid));
+    }
+  }
+  return false;
+}
+
 bool ParseMediaDescription(
     absl::string_view message,
     const TransportDescription& session_td,
@@ -2852,6 +2868,11 @@ bool ParseMediaDescription(
     // Create TransportInfo with the media level "ice-pwd" and "ice-ufrag".
     desc->AddTransportInfo(TransportInfo(content_name, transport));
   }
+  // Apply whole-description sanity checks
+  if (HasDuplicateMsidLines(desc)) {
+    ParseFailed(message, *pos, "Duplicate a=msid lines detected", error);
+    return false;
+  }
 
   desc->set_msid_signaling(msid_signaling);
 
diff --git a/pc/webrtc_sdp_unittest.cc b/pc/webrtc_sdp_unittest.cc
index a31aa2adef..999f0b6b26 100644
--- a/pc/webrtc_sdp_unittest.cc
+++ b/pc/webrtc_sdp_unittest.cc
@@ -4271,6 +4271,27 @@ TEST_F(WebRtcSdpTest, DeserializeMsidAttributeWithMissingStreamId) {
   EXPECT_FALSE(SdpDeserialize(sdp, &jdesc_output));
 }
 
+TEST_F(WebRtcSdpTest, DeserializeMsidAttributeWithDuplicateStreamIdAndTrackId) {
+  std::string sdp =
+      "v=0\r\n"
+      "o=- 18446744069414584320 18446462598732840960 IN IP4 127.0.0.1\r\n"
+      "s=-\r\n"
+      "t=0 0\r\n"
+      "m=audio 9 RTP/SAVPF 111\r\n"
+      "a=mid:0\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=rtpmap:111 opus/48000/2\r\n"
+      "a=msid:stream_id track_id\r\n"
+      "m=audio 9 RTP/SAVPF 111\r\n"
+      "a=mid:1\r\n"
+      "c=IN IP4 0.0.0.0\r\n"
+      "a=rtpmap:111 opus/48000/2\r\n"
+      "a=msid:stream_id track_id\r\n";
+
+  JsepSessionDescription jdesc_output(kDummyType);
+  EXPECT_FALSE(SdpDeserialize(sdp, &jdesc_output));
+}
+
 // Tests that if both session-level address and media-level address exist, use
 // the media-level address.
 TEST_F(WebRtcSdpTest, ParseConnectionData) {