diff --git a/tools/gn/ninja_build_writer.cc b/tools/gn/ninja_build_writer.cc
index a805bb9..74a7787 100644
--- a/tools/gn/ninja_build_writer.cc
+++ b/tools/gn/ninja_build_writer.cc
@@ -130,6 +130,21 @@
   return result;
 }
 
+// Given two toolchains with the same name, generates an error message
+// that describes the problem.
+Err GetDuplicateToolchainError(const SourceFile& source_file,
+                               const Toolchain* previous_toolchain,
+                               const Toolchain* toolchain) {
+  Err result(toolchain->defined_from(), "Duplicate toolchain.",
+      "Two or more toolchains write to the same directory:\n  " +
+      source_file.GetDir().value() + "\n\n"
+      "This can be fixed by making sure that distinct toolchains have\n"
+      "distinct names.\n");
+  result.AppendSubErr(
+      Err(previous_toolchain->defined_from(), "Previous toolchain."));
+  return result;
+}
+
 }  // namespace
 
 NinjaBuildWriter::NinjaBuildWriter(
@@ -156,8 +171,7 @@
 bool NinjaBuildWriter::Run(Err* err) {
   WriteNinjaRules();
   WriteAllPools();
-  WriteSubninjas();
-  return WritePhonyAndAllRules(err);
+  return WriteSubninjas(err) && WritePhonyAndAllRules(err);
 }
 
 // static
@@ -287,7 +301,7 @@
   }
 }
 
-void NinjaBuildWriter::WriteSubninjas() {
+bool NinjaBuildWriter::WriteSubninjas(Err* err) {
   // Write toolchains sorted by their name, to make output deterministic.
   std::vector<std::pair<const Settings*, const Toolchain*>> sorted_settings(
       used_toolchains_.begin(), used_toolchains_.end());
@@ -302,12 +316,29 @@
               return GetNinjaFileForToolchain(a.first) <
                      GetNinjaFileForToolchain(b.first);
             });
+
+  SourceFile previous_subninja;
+  const Toolchain* previous_toolchain = nullptr;
+
   for (const auto& pair : sorted_settings) {
+    SourceFile subninja = GetNinjaFileForToolchain(pair.first);
+
+    // Since the toolchains are sorted, comparing to the previous subninja is
+    // enough to find duplicates.
+    if (subninja == previous_subninja) {
+      *err =
+          GetDuplicateToolchainError(subninja, previous_toolchain, pair.second);
+      return false;
+    }
+
     out_ << "subninja ";
-    path_output_.WriteFile(out_, GetNinjaFileForToolchain(pair.first));
+    path_output_.WriteFile(out_, subninja);
     out_ << std::endl;
+    previous_subninja = subninja;
+    previous_toolchain = pair.second;
   }
   out_ << std::endl;
+  return true;
 }
 
 bool NinjaBuildWriter::WritePhonyAndAllRules(Err* err) {
diff --git a/tools/gn/ninja_build_writer.h b/tools/gn/ninja_build_writer.h
index 530ac71..51535aa 100644
--- a/tools/gn/ninja_build_writer.h
+++ b/tools/gn/ninja_build_writer.h
@@ -49,9 +49,8 @@
  private:
   void WriteNinjaRules();
   void WriteAllPools();
-  void WriteSubninjas();
-  bool WritePhonyAndAllRules(
-      Err* err);
+  bool WriteSubninjas(Err* err);
+  bool WritePhonyAndAllRules(Err* err);
 
   void WritePhonyRule(const Target* target, const std::string& phony_name);