diff --git a/clang/include/clang/3C/RewriteUtils.h b/clang/include/clang/3C/RewriteUtils.h
index 5fd4dc8de7c6..0f312418bf5e 100644
--- a/clang/include/clang/3C/RewriteUtils.h
+++ b/clang/include/clang/3C/RewriteUtils.h
@@ -159,6 +159,12 @@ class RewriteConsumer : public ASTConsumer {
   static std::set<PersistentSourceLoc> EmittedDiagnostics;
 
   void emitRootCauseDiagnostics(ASTContext &Context);
+
+  // Hack to avoid printing the main file to stdout multiple times in the edge
+  // case of a compilation database containing multiple translation units for
+  // the main file
+  // (https://github.com/correctcomputation/checkedc-clang/issues/374#issuecomment-893612654).
+  bool StdoutModeEmittedMainFile = false;
 };
 
 bool canRewrite(Rewriter &R, const SourceRange &SR);
diff --git a/clang/lib/3C/RewriteUtils.cpp b/clang/lib/3C/RewriteUtils.cpp
index b9eb8169b8e2..31656e9c80da 100644
--- a/clang/lib/3C/RewriteUtils.cpp
+++ b/clang/lib/3C/RewriteUtils.cpp
@@ -120,12 +120,11 @@ void rewriteSourceRange(Rewriter &R, const CharSourceRange &Range,
   }
 }
 
-static void emit(Rewriter &R, ASTContext &C) {
+static void emit(Rewriter &R, ASTContext &C, bool &StdoutModeEmittedMainFile) {
   if (Verbose)
     errs() << "Writing files out\n";
 
   bool StdoutMode = (OutputPostfix == "-" && OutputDir.empty());
-  bool StdoutModeSawMainFile = false;
   SourceManager &SM = C.getSourceManager();
   // Iterate over each modified rewrite buffer.
   for (auto Buffer = R.buffer_begin(); Buffer != R.buffer_end(); ++Buffer) {
@@ -226,9 +225,15 @@ static void emit(Rewriter &R, ASTContext &C) {
 
       if (StdoutMode) {
         if (Buffer->first == SM.getMainFileID()) {
-          // This is the new version of the main file. Print it to stdout.
-          Buffer->second.write(outs());
-          StdoutModeSawMainFile = true;
+          // This is the new version of the main file. Print it to stdout,
+          // except in the edge case where we have a compilation database with
+          // multiple translation units with the same main file and we already
+          // emitted a copy of the main file for a previous translation unit
+          // (https://github.com/correctcomputation/checkedc-clang/issues/374#issuecomment-893612654).
+          if (!StdoutModeEmittedMainFile) {
+            Buffer->second.write(outs());
+            StdoutModeEmittedMainFile = true;
+          }
         } else {
           unsigned ID = DE.getCustomDiagID(
               UnwritableChangeDiagnosticLevel,
@@ -300,9 +305,10 @@ static void emit(Rewriter &R, ASTContext &C) {
     }
   }
 
-  if (StdoutMode && !StdoutModeSawMainFile) {
+  if (StdoutMode && !StdoutModeEmittedMainFile) {
     // The main file is unchanged. Write out its original content.
     outs() << SM.getBufferOrFake(SM.getMainFileID()).getBuffer();
+    StdoutModeEmittedMainFile = true;
   }
 }
 
@@ -632,7 +638,7 @@ void RewriteConsumer::HandleTranslationUnit(ASTContext &Context) {
   }
 
   // Output files.
-  emit(R, Context);
+  emit(R, Context, StdoutModeEmittedMainFile);
 
   Info.getPerfStats().endRewritingTime();
 
diff --git a/clang/test/3C/multiple_tu.c b/clang/test/3C/multiple_tu.c
index d7fc64f5145c..f56cb54e6945 100644
--- a/clang/test/3C/multiple_tu.c
+++ b/clang/test/3C/multiple_tu.c
@@ -13,10 +13,22 @@
 // present in the root cause statistics json output. The json is used to
 // generate the output for 3c-wrap root_cause, so the error appeared there as well.
 
+// Furthermore, in stdout mode, if the main file was processed twice, the
+// rewritten version was printed to stdout twice
+// (https://github.com/correctcomputation/checkedc-clang/issues/374#issuecomment-893612654).
+// Check that this doesn't happen any more.
+
 _Itype_for_any(T) void my_free(void *pointer : itype(_Array_ptr<T>) byte_count(0));
 
 void foo() {
+  //CHECK: {{^}}void foo() {
   int *a;
   my_free(a);
   //CHECK: my_free<int>(a);
 }
+
+// Make sure the file does not get printed to stdout a second time. Since
+// -match-full-lines does not apply to CHECK-NOT, the {{^}} is needed to anchor
+// the match to the beginning of the line and prevent the CHECK-NOT from
+// matching itself.
+//CHECK-NOT: {{^}}void foo() {