diff --git a/clang/lib/CodeGen/BackendUtil.cpp b/clang/lib/CodeGen/BackendUtil.cpp
index 648c7b3df8ed47..510f3911939cf2 100644
--- a/clang/lib/CodeGen/BackendUtil.cpp
+++ b/clang/lib/CodeGen/BackendUtil.cpp
@@ -1034,8 +1034,9 @@ void EmitAssemblyHelper::EmitAssemblyWithLegacyPassManager(
         if (!ThinLinkOS)
           return;
       }
-      TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
-                               CodeGenOpts.EnableSplitLTOUnit);
+      if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
+        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                 CodeGenOpts.EnableSplitLTOUnit);
       PerModulePasses.add(createWriteThinLTOBitcodePass(
           *OS, ThinLinkOS ? &ThinLinkOS->os() : nullptr));
     } else {
@@ -1049,8 +1050,9 @@ void EmitAssemblyHelper::EmitAssemblyWithLegacyPassManager(
       if (EmitLTOSummary) {
         if (!TheModule->getModuleFlag("ThinLTO"))
           TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
-        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
-                                 uint32_t(1));
+        if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
+          TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                   uint32_t(1));
       }
 
       PerModulePasses.add(createBitcodeWriterPass(
@@ -1451,8 +1453,9 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
         if (!ThinLinkOS)
           return;
       }
-      TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
-                               CodeGenOpts.EnableSplitLTOUnit);
+      if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
+        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                 CodeGenOpts.EnableSplitLTOUnit);
       MPM.addPass(ThinLTOBitcodeWriterPass(*OS, ThinLinkOS ? &ThinLinkOS->os()
                                                            : nullptr));
     } else {
@@ -1465,8 +1468,9 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
       if (EmitLTOSummary) {
         if (!TheModule->getModuleFlag("ThinLTO"))
           TheModule->addModuleFlag(Module::Error, "ThinLTO", uint32_t(0));
-        TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
-                                 uint32_t(1));
+        if (!TheModule->getModuleFlag("EnableSplitLTOUnit"))
+          TheModule->addModuleFlag(Module::Error, "EnableSplitLTOUnit",
+                                   uint32_t(1));
       }
       MPM.addPass(
           BitcodeWriterPass(*OS, CodeGenOpts.EmitLLVMUseLists, EmitLTOSummary));
diff --git a/clang/test/CodeGen/enable-split-lto-unit.ll b/clang/test/CodeGen/enable-split-lto-unit.ll
new file mode 100644
index 00000000000000..2ca09a1ae3a8a1
--- /dev/null
+++ b/clang/test/CodeGen/enable-split-lto-unit.ll
@@ -0,0 +1,25 @@
+; Test that we do not duplicate the EnableSplitLTOUnit module flag.
+;
+; Disable the verifier so the compiler doesn't abort and thus lead to empty
+; output and false pass.
+;
+; RUN: %clang_cc1 -fno-legacy-pass-manager -emit-llvm-bc -flto=full -disable-llvm-verifier -o - %s | llvm-dis | FileCheck %s --check-prefix=FULL-NPM
+; RUN: %clang_cc1 -fno-legacy-pass-manager -emit-llvm-bc -flto=thin -disable-llvm-verifier -o - %s | llvm-dis | FileCheck %s --check-prefix=THIN-NPM
+; RUN: %clang_cc1 -flegacy-pass-manager -emit-llvm-bc -flto=full -disable-llvm-verifier -o - %s | llvm-dis | FileCheck %s --check-prefix=FULL-OPM
+; RUN: %clang_cc1 -flegacy-pass-manager -emit-llvm-bc -flto=thin -disable-llvm-verifier -o - %s | llvm-dis | FileCheck %s --check-prefix=THIN-OPM
+
+define dso_local void @main() local_unnamed_addr {
+entry:
+  ret void
+}
+
+; FULL-NPM-NOT: !llvm.module.flags = !{!0, !1, !2, !3, !3}
+; FULL-OPM-NOT: !llvm.module.flags = !{!0, !1, !2, !3, !3}
+; THIN-NPM-NOT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
+; THIN-OPM-NOT: !llvm.module.flags = !{!0, !1, !2, !3, !4}
+!llvm.module.flags = !{!0, !1, !2, !3}
+
+!0 = !{i32 1, !"wchar_size", i32 2}
+!1 = !{i32 7, !"frame-pointer", i32 2}
+!2 = !{i32 1, !"ThinLTO", i32 0}
+!3 = !{i32 1, !"EnableSplitLTOUnit", i32 1}