Do not overwrite already optimized code.

src/optimizing-compiler-thread.cc

Index: src/optimizing-compiler-thread.cc
diff --git a/src/optimizing-compiler-thread.cc b/src/optimizing-compiler-thread.cc
index d21507084c5f9d0de5d3eba72831f0f464665329..fb3eac5d503c1afd47be21cdc587c9f3ec097438 100644
--- a/src/optimizing-compiler-thread.cc
+++ b/src/optimizing-compiler-thread.cc
@@ -258,9 +258,13 @@ void OptimizingCompilerThread::InstallOptimizedFunctions() {
       uint32_t offset = code->TranslateAstIdToPcOffset(info->osr_ast_id());
       BackEdgeTable::RemoveStackCheck(code, offset);
     } else {
-      Handle<Code> code = Compiler::GetConcurrentlyOptimizedCode(job);
-      function->ReplaceCode(
-          code.is_null() ? function->shared()->code() : *code);
+      if (function->IsOptimized()) {
+        DisposeOptimizedCompileJob(job, false);
+      } else {
+        Handle<Code> code = Compiler::GetConcurrentlyOptimizedCode(job);
+        function->ReplaceCode(
+            code.is_null() ? function->shared()->code() : *code);
+      }
     }
   }
 }