--- a/dom/media/Benchmark.cpp
+++ b/dom/media/Benchmark.cpp
@@ -200,16 +200,20 @@ BenchmarkPlayback::InitDecoder(TrackInfo
     });
 }
 
 void
 BenchmarkPlayback::MainThreadShutdown()
 {
   MOZ_ASSERT(OnThread());
 
+  if (mFinished) {
+    // Nothing more to do.
+    return;
+  }
   mFinished = true;
 
   if (mDecoder) {
     mDecoder->Flush();
     mDecoder->Shutdown();
     mDecoder = nullptr;
   }