Reject re-inserting already decoded frames into FrameBuffer2.

webrtc/modules/video_coding/frame_buffer2_unittest.cc

Index: webrtc/modules/video_coding/frame_buffer2_unittest.cc
diff --git a/webrtc/modules/video_coding/frame_buffer2_unittest.cc b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
index 04ce5595a553519b2ad835db52e1da8820b931e5..4f1c6369dc276c4b3f3a2559928429080bdbb977 100644
--- a/webrtc/modules/video_coding/frame_buffer2_unittest.cc
+++ b/webrtc/modules/video_coding/frame_buffer2_unittest.cc
@@ -516,5 +516,11 @@ TEST_F(TestFrameBuffer2, ForwardJumps) {
   ExtractFrame();
 }
 
+TEST_F(TestFrameBuffer2, DuplicateFrames) {
+  EXPECT_EQ(22256, InsertFrame(22256, 0, 1, false));
+  ExtractFrame();
+  EXPECT_EQ(22256, InsertFrame(22256, 0, 1, false));
+}
+
 }  // namespace video_coding
 }  // namespace webrtc