diff --git a/lld/MachO/SymbolTable.cpp b/lld/MachO/SymbolTable.cpp
--- a/lld/MachO/SymbolTable.cpp
+++ b/lld/MachO/SymbolTable.cpp
@@ -59,31 +59,28 @@
   assert(!isWeakDef || (isa<BitcodeFile>(file) && !isec) ||
          (isa<ObjFile>(file) && file == isec->getFile()));
 
+  InputSection *curIsec = nullptr;
   if (!wasInserted) {
     if (auto *defined = dyn_cast<Defined>(s)) {
       if (isWeakDef) {
-        if (defined->isWeakDef()) {
-          // Both old and new symbol weak (e.g. inline function in two TUs):
-          // If one of them isn't private extern, the merged symbol isn't.
+        if (defined->isWeakDef())
           defined->privateExtern &= isPrivateExtern;
-          defined->referencedDynamically |= isReferencedDynamically;
-          defined->noDeadStrip |= noDeadStrip;
-
-          // FIXME: Handle this for bitcode files.
-          // FIXME: We currently only do this if both symbols are weak.
-          //        We could do this if either is weak (but getting the
-          //        case where !isWeakDef && defined->isWeakDef() right
-          //        requires some care and testing).
-          if (auto concatIsec = dyn_cast_or_null<ConcatInputSection>(isec))
-            concatIsec->wasCoalesced = true;
-        }
 
+        defined->referencedDynamically |= isReferencedDynamically;
+        defined->noDeadStrip |= noDeadStrip;
+
+        if (auto concatIsec = dyn_cast_or_null<ConcatInputSection>(isec))
+          concatIsec->wasCoalesced = true;
         return defined;
       }
+
       if (!defined->isWeakDef())
         error("duplicate symbol: " + name + "\n>>> defined in " +
               toString(defined->getFile()) + "\n>>> defined in " +
               toString(file));
+      else
+        curIsec = defined->isec;
+
     } else if (auto *dysym = dyn_cast<DylibSymbol>(s)) {
       overridesWeakDef = !isWeakDef && dysym->isWeakDef();
       dysym->unreference();
@@ -96,6 +93,10 @@
       s, name, file, isec, value, size, isWeakDef, /*isExternal=*/true,
       isPrivateExtern, isThumb, isReferencedDynamically, noDeadStrip);
   defined->overridesWeakDef = overridesWeakDef;
+  if (curIsec)
+    if (auto concatIsec = dyn_cast_or_null<ConcatInputSection>(curIsec))
+      concatIsec->wasCoalesced = true;
+
   return defined;
 }
 
diff --git a/lld/test/MachO/dup-symbols-weak-def.s b/lld/test/MachO/dup-symbols-weak-def.s
new file mode 100644
--- /dev/null
+++ b/lld/test/MachO/dup-symbols-weak-def.s
@@ -0,0 +1,48 @@
+# REQUIRES: x86
+# RUN: rm -rf %t; split-file %s %t
+
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos  %t/lib_def.s -o %t/lib_def.o
+# RUN: llvm-mc -filetype=obj -triple=x86_64-apple-macos %t/lib_weak.s -o %t/lib_weak.o
+# RUN: %lld -dylib -lSystem -dynamic  %t/lib_def.o %t/lib_weak.o -o %t/out.dylib
+# RUN: llvm-objdump --macho --syms %t/out.dylib | FileCheck %s
+
+# CHECK: SYMBOL TABLE:
+# CHECK-NEXT: 0{{.*}} g     F __TEXT,__text _uprv_getICUData_core
+
+        
+#--- lib_def.s
+.section __TEXT,__text,regular,pure_instructions
+
+.globl  _uprv_getICUData_core
+
+_uprv_getICUData_core:
+  .cfi_startproc
+        pushq   %rbp
+        .cfi_def_cfa_offset 16
+        .cfi_offset %rbp, -16
+        movq    %rsp, %rbp
+        .cfi_def_cfa_register %rbp
+        popq    %rbp
+        retq
+        .cfi_endproc
+
+.subsections_via_symbols
+
+#--- lib_weak.s
+.section        __TEXT,__text,regular,pure_instructions
+
+.globl  _uprv_getICUData_core
+.weak_definition _uprv_getICUData_core
+
+_uprv_getICUData_core:
+  .cfi_startproc
+        pushq   %rbp
+        .cfi_def_cfa_offset 16
+        .cfi_offset %rbp, -16
+        movq    %rsp, %rbp
+        .cfi_def_cfa_register %rbp
+        popq    %rbp
+        retq
+        .cfi_endproc
+
+.subsections_via_symbols