Index: ELF/LinkerScript.h
===================================================================
--- ELF/LinkerScript.h
+++ ELF/LinkerScript.h
@@ -104,6 +104,9 @@
   bool Provide = false;
   bool Hidden = false;
 
+  // Flag shows if this command is -defsym assignment.
+  bool IsDefsym = false;
+
   // Holds file name and line number for error reporting.
   std::string Location;
 };
Index: ELF/LinkerScript.cpp
===================================================================
--- ELF/LinkerScript.cpp
+++ ELF/LinkerScript.cpp
@@ -129,9 +129,6 @@
   // Define a symbol.
   Symbol *Sym;
   uint8_t Visibility = Cmd->Hidden ? STV_HIDDEN : STV_DEFAULT;
-  std::tie(Sym, std::ignore) = Symtab->insert(Cmd->Name, /*Type*/ 0, Visibility,
-                                              /*CanOmitFromDynSym*/ false,
-                                              /*File*/ nullptr);
   ExprValue Value = Cmd->Expression();
   SectionBase *Sec = Value.isAbsolute() ? nullptr : Value.Sec;
 
@@ -148,8 +145,21 @@
   // write expressions like this: `alignment = 16; . = ALIGN(., alignment)`.
   uint64_t SymValue = Value.Sec ? 0 : Value.getValue();
 
-  replaceSymbol<Defined>(Sym, nullptr, Cmd->Name, STB_GLOBAL, Visibility,
-                         STT_NOTYPE, SymValue, 0, Sec);
+  // Regular linker script assignment may be used to update existent symbol
+  // value, but --defsym assignment should always create new symbol and report
+  // duplicate symbol definition error if symbol already exists.
+  if (Cmd->IsDefsym) {
+    Sym = Symtab->addRegular(Cmd->Name, Visibility, STT_NOTYPE, SymValue,
+                             /*Size=*/0, STB_GLOBAL, Sec,
+                             /*File=*/nullptr);
+  } else {
+    std::tie(Sym, std::ignore) =
+        Symtab->insert(Cmd->Name, /*Type*/ 0, Visibility,
+                       /*CanOmitFromDynSym*/ false,
+                       /*File*/ nullptr);
+    replaceSymbol<Defined>(Sym, nullptr, Cmd->Name, STB_GLOBAL, Visibility,
+                           STT_NOTYPE, SymValue, 0, Sec);
+  }
   Cmd->Sym = cast<Defined>(Sym);
 }
 
Index: ELF/ScriptParser.cpp
===================================================================
--- ELF/ScriptParser.cpp
+++ ELF/ScriptParser.cpp
@@ -277,6 +277,7 @@
   if (!atEOF())
     setError("EOF expected, but got " + next());
   SymbolAssignment *Cmd = make<SymbolAssignment>(Name, E, getCurrentLocation());
+  Cmd->IsDefsym = true;
   Script->SectionCommands.push_back(Cmd);
 }
 
Index: test/ELF/linkerscript/symbol-multiple-defs.s
===================================================================
--- test/ELF/linkerscript/symbol-multiple-defs.s
+++ test/ELF/linkerscript/symbol-multiple-defs.s
@@ -0,0 +1,14 @@
+# REQUIRES: x86
+# RUN: llvm-mc -filetype=obj -triple=x86_64-unknown-linux %s -o %t
+
+# RUN: echo "foo = 0x3;" > %t.script
+# RUN: ld.lld -o %t1 --script %t.script %t
+# RUN: llvm-objdump -t %t1 | FileCheck %s --check-prefix=SCRIPT
+# SCRIPT: 0000000000000003 *ABS* 00000000 foo
+
+# RUN: not ld.lld --defsym=foo=0x3 -o %t2 %t 2>&1 \
+# RUN:  | FileCheck %s --check-prefix=ERR
+# ERR: duplicate symbol: foo
+
+.global foo
+foo: