diff --git a/input_sections.cc b/input_sections.cc
index af71bf84..228388d6 100644
--- a/input_sections.cc
+++ b/input_sections.cc
@@ -148,7 +148,11 @@ void InputSection::scan_relocations() {
   for (int i = 0; i < rels.size(); i++) {
     const ELF64LE::Rela &rel = rels[i];
     Symbol &sym = *file->symbols[rel.getSymbol(false)];
-    assert(sym.file);
+
+    if (!sym.file || sym.is_placeholder) {
+      file->has_error = true;
+      continue;
+    }
 
     switch (rel.getType(false)) {
     case R_X86_64_NONE:
@@ -207,6 +211,18 @@ void InputSection::scan_relocations() {
   }
 }
 
+void InputSection::report_undefined_symbols() {
+  if (!(shdr.sh_flags & SHF_ALLOC))
+    return;
+
+  for (const ELF64LE::Rela &rel : rels) {
+    Symbol &sym = *file->symbols[rel.getSymbol(false)];
+    if (!sym.file || sym.is_placeholder)
+      llvm::errs() << "undefined symbol: " << toString(file)
+                   << ": " << sym.name << "\n";
+  }
+}
+
 MergeableSection::MergeableSection(InputSection *isec, ArrayRef<u8> contents)
   : InputChunk(MERGEABLE, isec->file, isec->shdr, isec->name),
     parent(*MergedSection::get_instance(isec->name, isec->shdr.sh_flags,
diff --git a/main.cc b/main.cc
index 462aea53..a1872693 100644
--- a/main.cc
+++ b/main.cc
@@ -301,50 +301,34 @@ static void bin_sections() {
   });
 }
 
-static void check_undefined_symbols() {
+static void check_duplicate_symbols() {
   MyTimer t("check_undef_syms", before_copy_timer);
-  std::vector<bool> has_errors(out::objs.size());
 
-  tbb::parallel_for(0, (int)out::objs.size(), [&](int i) {
-    ObjectFile *file = out::objs[i];
-    if (!file->is_alive || file->is_dso)
+  tbb::parallel_for_each(out::objs, [](ObjectFile *file) {
+    if (!file->is_alive)
       return;
 
-    for (int j = file->first_global; j < file->elf_syms.size(); j++) {
-      const ELF64LE::Sym &esym = file->elf_syms[j];
-      Symbol &sym = *file->symbols[j];
+    for (int i = file->first_global; i < file->elf_syms.size(); i++) {
+      const ELF64LE::Sym &esym = file->elf_syms[i];
+      Symbol &sym = *file->symbols[i];
       bool is_weak = (esym.getBinding() == STB_WEAK);
 
-      if (esym.isUndefined() && !is_weak && (!sym.file || sym.is_placeholder)) {
-        has_errors[i] = true;
-        return;
-      }
-
       if (esym.isDefined() && !is_weak && sym.file != file) {
-        has_errors[i] = true;
+        file->has_error = true;
         return;
       }
     }
   });
 
-  bool has_error = false;
-
-  for (int i = 0; i < out::objs.size(); i++) {
-    if (!has_errors[i])
+  for (ObjectFile *file : out::objs) {
+    if (!file->has_error)
       continue;
-    has_error = true;
 
-    ObjectFile *file = out::objs[i];
-
-    for (int j = file->first_global; j < file->elf_syms.size(); j++) {
-      const ELF64LE::Sym &esym = file->elf_syms[j];
-      Symbol &sym = *file->symbols[j];
+    for (int i = file->first_global; i < file->elf_syms.size(); i++) {
+      const ELF64LE::Sym &esym = file->elf_syms[i];
+      Symbol &sym = *file->symbols[i];
       bool is_weak = (esym.getBinding() == STB_WEAK);
 
-      if (esym.isUndefined() && !is_weak && (!sym.file || sym.is_placeholder))
-        llvm::errs() << "undefined symbol: " << toString(file)
-                     << ": " << sym.name << "\n";
-
       if (esym.isDefined() && !is_weak && sym.file != file)
         llvm::errs() << "duplicate symbol: " << toString(file)
                      << ": " << toString(sym.file) << ": "
@@ -352,8 +336,9 @@ static void check_undefined_symbols() {
     }
   }
 
-  if (has_error)
-    exit(1);
+  for (ObjectFile *file : out::objs)
+    if (file->has_error)
+      _exit(1);
 }
 
 static void set_isec_offsets() {
@@ -407,6 +392,16 @@ static void scan_rels() {
         isec->scan_relocations();
   });
 
+  for (ObjectFile *file : out::objs)
+    if (file->has_error)
+      for (InputSection *isec : file->sections)
+        if (isec)
+          isec->report_undefined_symbols();
+
+  for (ObjectFile *file : out::objs)
+    if (file->has_error)
+      _exit(1);
+
   std::vector<InputFile *> files;
   files.insert(files.end(), out::objs.begin(), out::objs.end());
   files.insert(files.end(), out::dsos.begin(), out::dsos.end());
@@ -876,9 +871,6 @@ int main(int argc, char **argv) {
 
   // Beyond this point, no new symbols will be added to the result.
 
-  // Make sure that all symbols have been resolved.
-  check_undefined_symbols();
-
   // Copy shared object name strings to .dynsym
   for (SharedFile *file : out::dsos)
     out::dynstr->add_string(file->soname);
@@ -896,6 +888,9 @@ int main(int argc, char **argv) {
     if (out::chunks[i]->kind != OutputChunk::HEADER)
       out::chunks[i]->shndx = shndx++;
 
+  // Make sure that all symbols have been resolved.
+  check_duplicate_symbols();
+
   // Scan relocations to fix the sizes of .got, .plt, .got.plt, .dynstr,
   // .rela.dyn, .rela.plt.
   scan_rels();
diff --git a/mold.h b/mold.h
index bed5ca86..7bfd2086 100644
--- a/mold.h
+++ b/mold.h
@@ -261,7 +261,6 @@ class InputChunk {
 public:
   enum Kind : u8 { REGULAR, MERGEABLE };
 
-  virtual void scan_relocations() {}
   virtual void copy_buf() {}
   u64 get_addr() const;
 
@@ -283,7 +282,8 @@ public:
     : InputChunk(REGULAR, file, shdr, name) {}
 
   void copy_buf() override;
-  void scan_relocations() override;
+  void scan_relocations();
+  void report_undefined_symbols();
 
   ArrayRef<ELF64LE::Rela> rels;
   std::vector<StringPieceRef> rel_pieces;
@@ -732,6 +732,7 @@ public:
   ArrayRef<ELF64LE::Sym> elf_syms;
   int first_global = 0;
   const bool is_in_archive;
+  std::atomic_bool has_error = ATOMIC_VAR_INIT(false);
 
   u64 local_symtab_size = 0;
   u64 local_strtab_size = 0;
diff --git a/object_file.cc b/object_file.cc
index 5b9c806c..d305ee47 100644
--- a/object_file.cc
+++ b/object_file.cc
@@ -519,6 +519,7 @@ void ObjectFile::handle_undefined_weak_symbols() {
         sym.value = 0;
         sym.visibility = esym.getVisibility();
         sym.esym = &esym;
+        sym.is_placeholder = false;
         sym.is_undef_weak = true;
         sym.is_imported = false;
 
diff --git a/test/duplicate-error.s b/test/duplicate-error.s
new file mode 100644
index 00000000..ee070bc9
--- /dev/null
+++ b/test/duplicate-error.s
@@ -0,0 +1,8 @@
+// RUN: cc -o %t.o -c %s
+// RUN: not mold -o %t.exe %t.o %t.o 2> %t.log
+// RUN: grep 'duplicate symbol: .*\.o: .*\.o: main' %t.log
+
+        .text
+        .globl main
+main:
+        nop
diff --git a/test/missing-but-ok.s b/test/missing-but-ok.s
new file mode 100644
index 00000000..617b7371
--- /dev/null
+++ b/test/missing-but-ok.s
@@ -0,0 +1,7 @@
+// RUN: cc -o %t.o -c %s
+// RUN: mold -o %t.exe %t.o
+
+        .text
+        .globl _start, foo
+_start:
+        nop
diff --git a/test/missing-error.s b/test/missing-error.s
new file mode 100644
index 00000000..46be01a2
--- /dev/null
+++ b/test/missing-error.s
@@ -0,0 +1,8 @@
+// RUN: cc -o %t.o -c %s
+// RUN: not mold -o %t.exe %t.o 2> %t.log
+// RUN: grep 'undefined symbol: .*\.o: foo' %t.log
+
+        .text
+        .globl main
+main:
+        call foo