diff --git a/caffe2/serialize/inline_container.cc b/caffe2/serialize/inline_container.cc
index d74dfb4a4a1c..cb36fe0a4bc1 100644
--- a/caffe2/serialize/inline_container.cc
+++ b/caffe2/serialize/inline_container.cc
@@ -9,6 +9,7 @@
 #include <c10/core/Allocator.h>
 #include <c10/core/CPUAllocator.h>
 #include <c10/core/Backend.h>
+#include <c10/util/Exception.h>
 
 #include "caffe2/core/common.h"
 #include "caffe2/core/logging.h"
@@ -235,8 +236,9 @@ std::vector<std::string> PyTorchStreamReader::getAllRecords() {
   return out;
 }
 
-const std::vector<std::string>& PyTorchStreamWriter::getAllWrittenRecords() {
-  return files_written;
+const std::unordered_set<std::string>&
+PyTorchStreamWriter::getAllWrittenRecords() {
+  return files_written_;
 }
 
 size_t PyTorchStreamReader::getRecordID(const std::string& name) {
@@ -356,6 +358,8 @@ void PyTorchStreamWriter::writeRecord(
     bool compress) {
   AT_ASSERT(!finalized_);
   AT_ASSERT(!archive_name_plus_slash_.empty());
+  TORCH_INTERNAL_ASSERT(
+      files_written_.count(name) == 0, "Tried to serialize file twice: ", name);
   std::string full_name = archive_name_plus_slash_ + name;
   size_t padding_size =
       detail::getPadding(ar_->m_archive_size, full_name.size(), size, padding_);
@@ -376,7 +380,7 @@ void PyTorchStreamWriter::writeRecord(
       nullptr,
       0);
   valid("writing file ", name.c_str());
-  files_written.push_back(name);
+  files_written_.insert(name);
 }
 
 void PyTorchStreamWriter::writeEndOfFile() {
diff --git a/caffe2/serialize/inline_container.h b/caffe2/serialize/inline_container.h
index 281d1756d75f..4eb1b8e71ce6 100644
--- a/caffe2/serialize/inline_container.h
+++ b/caffe2/serialize/inline_container.h
@@ -7,6 +7,7 @@
 #include <istream>
 #include <mutex>
 #include <ostream>
+#include <unordered_set>
 
 #include <c10/core/Allocator.h>
 #include <c10/core/Backend.h>
@@ -140,7 +141,7 @@ class TORCH_API PyTorchStreamWriter final {
       bool compress = false);
   void writeEndOfFile();
 
-  const std::vector<std::string>& getAllWrittenRecords();
+  const std::unordered_set<std::string>& getAllWrittenRecords();
 
   bool finalized() const {
     return finalized_;
@@ -156,7 +157,7 @@ class TORCH_API PyTorchStreamWriter final {
   void setup(const std::string& file_name);
   void valid(const char* what, const char* info = "");
   size_t current_pos_ = 0;
-  std::vector<std::string> files_written;
+  std::unordered_set<std::string> files_written_;
   std::unique_ptr<mz_zip_archive> ar_;
   std::string archive_name_;
   std::string archive_name_plus_slash_;
@@ -184,7 +185,7 @@ size_t getPadding(
     size_t filename_size,
     size_t size,
     std::string& padding_buf);
-}
+} // namespace detail
 
 } // namespace serialize
 } // namespace caffe2
diff --git a/caffe2/serialize/inline_container_test.cc b/caffe2/serialize/inline_container_test.cc
index 3a9f511ee9cf..7a65bf1ab45d 100644
--- a/caffe2/serialize/inline_container_test.cc
+++ b/caffe2/serialize/inline_container_test.cc
@@ -35,9 +35,11 @@ TEST(PyTorchStreamWriterAndReader, SaveAndLoad) {
   }
   writer.writeRecord("key2", data2.data(), data2.size());
 
-  const std::vector<std::string>& written_records = writer.getAllWrittenRecords();
-  ASSERT_EQ(written_records[0], "key1");
-  ASSERT_EQ(written_records[1], "key2");
+  const std::unordered_set<std::string>& written_records =
+      writer.getAllWrittenRecords();
+  ASSERT_EQ(written_records.size(), 2);
+  ASSERT_EQ(written_records.count("key1"), 1);
+  ASSERT_EQ(written_records.count("key2"), 1);
 
   writer.writeEndOfFile();
 
@@ -95,9 +97,11 @@ TEST(PytorchStreamWriterAndReader, GetNonexistentRecordThrows) {
   }
   writer.writeRecord("key2", data2.data(), data2.size());
 
-  const std::vector<std::string>& written_records = writer.getAllWrittenRecords();
-  ASSERT_EQ(written_records[0], "key1");
-  ASSERT_EQ(written_records[1], "key2");
+  const std::unordered_set<std::string>& written_records =
+      writer.getAllWrittenRecords();
+  ASSERT_EQ(written_records.size(), 2);
+  ASSERT_EQ(written_records.count("key1"), 1);
+  ASSERT_EQ(written_records.count("key2"), 1);
 
   writer.writeEndOfFile();
 
diff --git a/torch/csrc/jit/mobile/backport_manager.cpp b/torch/csrc/jit/mobile/backport_manager.cpp
index 91c8548ee7d8..3cd815626c0f 100644
--- a/torch/csrc/jit/mobile/backport_manager.cpp
+++ b/torch/csrc/jit/mobile/backport_manager.cpp
@@ -244,7 +244,7 @@ void writeArchiveV5(
   std::string prefix = archive_name + "/";
 
   TORCH_INTERNAL_ASSERT(tensor_names.size() == data_pickle.tensorData().size());
-  const std::vector<std::string>& pre_serialized_files =
+  const std::unordered_set<std::string>& pre_serialized_files =
       writer.getAllWrittenRecords();
 
   for (const auto& td : data_pickle.tensorData()) {