diff --git a/lld/MachO/InputFiles.cpp b/lld/MachO/InputFiles.cpp
--- a/lld/MachO/InputFiles.cpp
+++ b/lld/MachO/InputFiles.cpp
@@ -779,7 +779,41 @@
   }
 }
 
+static macho::Symbol *createBitcodeSymbol(const lto::InputFile::Symbol &objSym,
+                                          BitcodeFile &file) {
+  StringRef name = saver.save(objSym.getName());
+
+  // TODO: support weak references
+  if (objSym.isUndefined())
+    return symtab->addUndefined(name, &file, /*isWeakRef=*/false);
+
+  assert(!objSym.isCommon() && "TODO: support common symbols in LTO");
+
+  // TODO: Write a test demonstrating why computing isPrivateExtern before
+  // LTO compilation is important.
+  bool isPrivateExtern = false;
+  switch (objSym.getVisibility()) {
+  case GlobalValue::HiddenVisibility:
+    isPrivateExtern = true;
+    break;
+  case GlobalValue::ProtectedVisibility:
+    error(name + " has protected visibility, which is not supported by Mach-O");
+    break;
+  case GlobalValue::DefaultVisibility:
+    break;
+  }
+
+  return symtab->addDefined(name, &file, /*isec=*/nullptr, /*value=*/0,
+                            objSym.isWeak(), isPrivateExtern);
+}
+
 BitcodeFile::BitcodeFile(MemoryBufferRef mbref)
     : InputFile(BitcodeKind, mbref) {
   obj = check(lto::InputFile::create(mbref));
+
+  // Convert LTO Symbols to LLD Symbols in order to perform resolution. The
+  // "winning" symbol will then be marked as Prevailing at LTO compilation
+  // time.
+  for (const lto::InputFile::Symbol &objSym : obj->symbols())
+    symbols.push_back(createBitcodeSymbol(objSym, *this));
 }
diff --git a/lld/MachO/LTO.cpp b/lld/MachO/LTO.cpp
--- a/lld/MachO/LTO.cpp
+++ b/lld/MachO/LTO.cpp
@@ -10,6 +10,7 @@
 #include "Config.h"
 #include "Driver.h"
 #include "InputFiles.h"
+#include "Symbols.h"
 
 #include "lld/Common/ErrorHandler.h"
 #include "lld/Common/Strings.h"
@@ -50,16 +51,24 @@
   resols.reserve(objSyms.size());
 
   // Provide a resolution to the LTO API for each symbol.
+  auto symIt = f.symbols.begin();
   for (const lto::InputFile::Symbol &objSym : objSyms) {
     resols.emplace_back();
     lto::SymbolResolution &r = resols.back();
+    Symbol *sym = *symIt++;
 
     // Ideally we shouldn't check for SF_Undefined but currently IRObjectFile
     // reports two symbols for module ASM defined. Without this check, lld
     // flags an undefined in IR with a definition in ASM as prevailing.
     // Once IRObjectFile is fixed to report only one symbol this hack can
     // be removed.
-    r.Prevailing = !objSym.isUndefined();
+    r.Prevailing = !objSym.isUndefined() && sym->getFile() == &f;
+
+    // Un-define the symbol so that we don't get duplicate symbol errors when we
+    // load the ObjFile emitted by LTO compilation.
+    if (r.Prevailing)
+      replaceSymbol<Undefined>(sym, sym->getName(), sym->getFile(),
+                               RefState::Strong);
 
     // TODO: set the other resolution configs properly
     r.VisibleToRegularObj = true;
diff --git a/lld/test/MachO/invalid/duplicate-symbol.ll b/lld/test/MachO/invalid/duplicate-symbol.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/invalid/duplicate-symbol.ll
@@ -0,0 +1,15 @@
+; REQUIRES: x86
+; RUN: rm -rf %t; mkdir -p %t
+; RUN: opt -module-summary %s -o %t/first.o
+; RUN: opt -module-summary %s -o %t/second.o
+; RUN: not %lld -dylib -lSystem %t/first.o %t/second.o -o /dev/null 2>&1 | FileCheck %s
+; CHECK:      error: duplicate symbol: _foo
+; CHECK-NEXT: >>> defined in {{.*}}/first.o
+; CHECK-NEXT: >>> defined in {{.*}}/second.o
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define void @foo() {
+  ret void
+}
diff --git a/lld/test/MachO/invalid/protected.ll b/lld/test/MachO/invalid/protected.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/invalid/protected.ll
@@ -0,0 +1,11 @@
+; REQUIRES: x86
+; RUN: opt -module-summary %s -o %t.o
+; RUN: not %lld -dylib -lSystem %t.o -o /dev/null 2>&1 | FileCheck %s
+; CHECK: error: _foo has protected visibility, which is not supported by Mach-O
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define protected void @foo() {
+  ret void
+}
diff --git a/lld/test/MachO/linkonce.ll b/lld/test/MachO/linkonce.ll
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/linkonce.ll
@@ -0,0 +1,29 @@
+; REQUIRES: x86
+; RUN: rm -rf %t; split-file %s %t
+; RUN: opt -module-summary %t/first.ll -o %t/first.o
+; RUN: opt -module-summary %t/second.ll -o %t/second.o
+; RUN: %lld -dylib -lSystem %t/first.o %t/second.o -o %t/12
+; RUN: llvm-objdump --syms %t/12 | FileCheck %s --check-prefix=FIRST
+; RUN: %lld -dylib -lSystem %t/second.o %t/first.o -o %t/21
+; RUN: llvm-objdump --syms %t/21 | FileCheck %s --check-prefix=SECOND
+
+; FIRST:  w    O __TEXT,first  _foo
+; SECOND: w    O __TEXT,second _foo
+
+#--- first.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define linkonce void @foo() section "__TEXT,first" {
+  ret void
+}
+
+#--- second.ll
+
+target triple = "x86_64-apple-macosx10.15.0"
+target datalayout = "e-m:o-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
+
+define linkonce void @foo() section "__TEXT,second" {
+  ret void
+}