Index: lld/MachO/ConcatOutputSection.cpp
===================================================================
--- lld/MachO/ConcatOutputSection.cpp
+++ lld/MachO/ConcatOutputSection.cpp
@@ -57,14 +57,14 @@
 // implement thunks. TODO: Adding support for branch islands!
 //
 // Internally -- as expressed in LLD's data structures -- a
-// branch-range-extension thunk comprises ...
+// branch-range-extension thunk consists of:
 //
-// (1) new Defined privateExtern symbol for the thunk named
+// (1) new Defined symbol for the thunk named
 //     <FUNCTION>.thunk.<SEQUENCE>, which references ...
 // (2) new InputSection, which contains ...
 // (3.1) new data for the instructions to load & branch to the far address +
 // (3.2) new Relocs on instructions to load the far address, which reference ...
-// (4.1) existing Defined extern symbol for the real function in __text, or
+// (4.1) existing Defined symbol for the real function in __text, or
 // (4.2) existing DylibSymbol for the real function in a dylib
 //
 // Nearly-optimal thunk-placement algorithm features:
@@ -89,8 +89,8 @@
 //   goes out of range, we increment the sequence number and create a new
 //   thunk named <FUNCTION>.thunk.<SEQUENCE>.
 //
-// * A thunk incarnation comprises (a) private-extern Defined symbol pointing
-//   to (b) an InputSection holding machine instructions (similar to a MachO
+// * A thunk consists of (a) a Defined symbol pointing to
+//   (b) an InputSection holding machine instructions (similar to a MachO
 //   stub), and (c) Reloc(s) that reference the real function for fixing-up
 //   the stub code.
 //
@@ -323,11 +323,20 @@
 
       StringRef thunkName = saver().save(funcSym->getName() + ".thunk." +
                                          std::to_string(thunkInfo.sequence++));
-      r.referent = thunkInfo.sym = symtab->addDefined(
-          thunkName, /*file=*/nullptr, thunkInfo.isec, /*value=*/0,
-          /*size=*/thunkSize, /*isWeakDef=*/false, /*isPrivateExtern=*/true,
-          /*isThumb=*/false, /*isReferencedDynamically=*/false,
-          /*noDeadStrip=*/false, /*isWeakDefCanBeHidden=*/false);
+      if (!isa<Defined>(funcSym) || cast<Defined>(funcSym)->isExternal()) {
+        r.referent = thunkInfo.sym = symtab->addDefined(
+            thunkName, /*file=*/nullptr, thunkInfo.isec, /*value=*/0,
+            thunkSize, /*isWeakDef=*/false, /*isPrivateExtern=*/true,
+            /*isThumb=*/false, /*isReferencedDynamically=*/false,
+            /*noDeadStrip=*/false, /*isWeakDefCanBeHidden=*/false);
+      } else {
+        r.referent = thunkInfo.sym = make<Defined>(
+            thunkName, /*file=*/nullptr, thunkInfo.isec, /*value=*/0,
+            thunkSize, /*isWeakDef=*/false, /*isExternal=*/false,
+            /*isPrivateExtern=*/true, /*isThumb=*/false,
+            /*isReferencedDynamically=*/false, /*noDeadStrip=*/false,
+            /*isWeakDefCanBeHidden=*/false);
+      }
       thunkInfo.sym->used = true;
       target->populateThunk(thunkInfo.isec, funcSym);
       finalizeOne(thunkInfo.isec);
Index: lld/test/MachO/arm64-thunk-starvation.s
===================================================================
--- lld/test/MachO/arm64-thunk-starvation.s
+++ lld/test/MachO/arm64-thunk-starvation.s
@@ -18,7 +18,7 @@
 ## Currently leaves 12 bytes for one thunk, so 36 bytes.
 ## Uses < instead of <=, so 40 bytes.
 
-.global _spacer1, _spacer1
+.global _spacer1, _spacer2
 ## 0x8000000 is 128 MiB, one more than the forward branch limit,
 ## distributed over two functions since our thunk insertion algorithm
 ## can't deal with a single function that's 128 MiB.
Index: lld/test/MachO/arm64-thunk-visibility.s
===================================================================
--- /dev/null
+++ lld/test/MachO/arm64-thunk-visibility.s
@@ -0,0 +1,83 @@
+# REQUIRES: aarch64
+
+# foo.s and bar.s both contain TU-local symbols (think static function)
+# with the same name, and both need a thunk..  This tests that ld64.lld doesn't
+# create a duplicate symbol for the two functions.
+
+# Test this both when the TU-local symbol is the branch source or target,
+# and for both forward and backwards jumps.
+
+# RUN: rm -rf %t; split-file %s %t
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/foo.s -o %t/foo.o
+# RUN: llvm-mc -filetype=obj -triple=arm64-apple-darwin %t/bar.s -o %t/bar.o
+# RUN: %lld -arch arm64 -lSystem -o %t.out %t/foo.o %t/bar.o
+
+#--- foo.s
+
+.subsections_via_symbols
+
+# Note: No .globl, since these are TU-local symbols.
+.p2align 2
+_early_jumping_local_fn: b _some_late_external
+.p2align 2
+_early_landing_local_fn: ret
+
+.globl _some_early_external
+.p2align 2
+_some_early_external: b _late_landing_local_fn
+
+## 0x8000000 is 128 MiB, one more than the forward branch limit.
+## Distribute that over two functions since our thunk insertion algorithm
+## can't deal with a single function that's 128 MiB.
+.global _spacer1, _spacer2
+_spacer1:
+.space 0x4000000
+_spacer2:
+.space 0x4000000
+
+# Note: No .globl, since these are TU-local symbols.
+.p2align 2
+_late_jumping_local_fn: b _some_early_external
+.p2align 2
+_late_landing_local_fn: ret
+
+.globl _some_late_external
+.p2align 2
+_some_late_external: b _early_landing_local_fn
+
+#--- bar.s
+
+.subsections_via_symbols
+
+# Note: No .globl, since these are TU-local symbols.
+.p2align 2
+_early_jumping_local_fn: b _some_other_late_external
+.p2align 2
+_early_landing_local_fn: ret
+
+.globl _some_other_early_external
+.p2align 2
+_some_other_early_external: b _late_landing_local_fn
+
+## 0x8000000 is 128 MiB, one more than the forward branch limit.
+## Distribute that over two functions since our thunk insertion algorithm
+## can't deal with a single function that's 128 MiB.
+.global _other_spacer1, _other_spacer1
+_spacer1:
+.space 0x4000000
+_spacer2:
+.space 0x4000000
+
+# Note: No .globl, since these are TU-local symbols.
+.p2align 2
+_late_jumping_local_fn: b _some_other_early_external
+.p2align 2
+_late_landing_local_fn: ret
+
+.globl _some_other_late_external
+.p2align 2
+_some_other_late_external: b _early_landing_local_fn
+
+.globl _main
+_main:
+  ret