diff --git a/client/client_test.gyp b/client/client_test.gyp
index 85cdb557..80cddc84 100644
--- a/client/client_test.gyp
+++ b/client/client_test.gyp
@@ -22,6 +22,7 @@
       'type': 'executable',
       'dependencies': [
         'client.gyp:crashpad_client',
+        '../handler/handler.gyp:crashpad_handler',
         '../test/test.gyp:crashpad_test',
         '../third_party/gtest/gmock.gyp:gmock',
         '../third_party/gtest/gmock.gyp:gmock_main',
@@ -35,6 +36,7 @@
       'sources': [
         'capture_context_mac_test.cc',
         'crash_report_database_test.cc',
+        'crashpad_client_win_test.cc',
         'prune_crash_reports_test.cc',
         'settings_test.cc',
         'simple_string_dictionary_test.cc',
diff --git a/client/crashpad_client_win.cc b/client/crashpad_client_win.cc
index 8faebbf2..2ef07a7e 100644
--- a/client/crashpad_client_win.cc
+++ b/client/crashpad_client_win.cc
@@ -132,7 +132,8 @@ using ScopedProcThreadAttributeList =
     base::ScopedGeneric<PPROC_THREAD_ATTRIBUTE_LIST,
                         ScopedProcThreadAttributeListTraits>;
 
-// Adds |handle| to |handle_list| if it appears valid.
+// Adds |handle| to |handle_list| if it appears valid, and is not already in
+// |handle_list|.
 //
 // Invalid handles (including INVALID_HANDLE_VALUE and null handles) cannot be
 // added to a PPROC_THREAD_ATTRIBUTE_LIST’s PROC_THREAD_ATTRIBUTE_HANDLE_LIST.
@@ -142,7 +143,12 @@ using ScopedProcThreadAttributeList =
 //
 // Use this function to add handles with uncertain validities.
 void AddHandleToListIfValid(std::vector<HANDLE>* handle_list, HANDLE handle) {
-  if (handle && handle != INVALID_HANDLE_VALUE) {
+  // There doesn't seem to be any documentation of this, but if there's a handle
+  // duplicated in this list, CreateProcess() fails with
+  // ERROR_INVALID_PARAMETER.
+  if (handle && handle != INVALID_HANDLE_VALUE &&
+      std::find(handle_list->begin(), handle_list->end(), handle) ==
+          handle_list->end()) {
     handle_list->push_back(handle);
   }
 }
@@ -337,6 +343,7 @@ bool CrashpadClient::UseHandler() {
   DCHECK_EQ(g_signal_non_crash_dump, INVALID_HANDLE_VALUE);
   DCHECK_EQ(g_non_crash_dump_done, INVALID_HANDLE_VALUE);
   DCHECK(!g_critical_section_with_debug_info.DebugInfo);
+  DCHECK(!g_non_crash_dump_lock);
 
   ClientToServerMessage message;
   memset(&message, 0, sizeof(message));
diff --git a/client/crashpad_client_win_test.cc b/client/crashpad_client_win_test.cc
new file mode 100644
index 00000000..2215e2a8
--- /dev/null
+++ b/client/crashpad_client_win_test.cc
@@ -0,0 +1,91 @@
+// Copyright 2015 The Crashpad Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "client/crashpad_client.h"
+
+#include "base/files/file_path.h"
+#include "gtest/gtest.h"
+#include "test/paths.h"
+#include "test/scoped_temp_dir.h"
+#include "test/win/win_multiprocess.h"
+
+namespace crashpad {
+namespace test {
+namespace {
+
+void StartAndUseHandler() {
+  ScopedTempDir temp_dir;
+  base::FilePath handler_path = Paths::Executable().DirName().Append(
+      FILE_PATH_LITERAL("crashpad_handler.exe"));
+  CrashpadClient client;
+  ASSERT_TRUE(client.StartHandler(handler_path,
+                                  temp_dir.path(),
+                                  "",
+                                  std::map<std::string, std::string>(),
+                                  std::vector<std::string>(),
+                                  false));
+  EXPECT_TRUE(client.UseHandler());
+}
+
+class StartWithInvalidHandles final : public WinMultiprocess {
+ public:
+  StartWithInvalidHandles() : WinMultiprocess() {}
+  ~StartWithInvalidHandles() {}
+
+ private:
+  void WinMultiprocessParent() override {}
+
+  void WinMultiprocessChild() override {
+    HANDLE original_stdout = GetStdHandle(STD_OUTPUT_HANDLE);
+    HANDLE original_stderr = GetStdHandle(STD_ERROR_HANDLE);
+    SetStdHandle(STD_OUTPUT_HANDLE, INVALID_HANDLE_VALUE);
+    SetStdHandle(STD_ERROR_HANDLE, INVALID_HANDLE_VALUE);
+
+    StartAndUseHandler();
+
+    SetStdHandle(STD_OUTPUT_HANDLE, original_stdout);
+    SetStdHandle(STD_ERROR_HANDLE, original_stderr);
+  }
+};
+
+TEST(CrashpadClient, StartWithInvalidHandles) {
+  WinMultiprocess::Run<StartWithInvalidHandles>();
+}
+
+class StartWithSameStdoutStderr final : public WinMultiprocess {
+ public:
+  StartWithSameStdoutStderr() : WinMultiprocess() {}
+  ~StartWithSameStdoutStderr() {}
+
+ private:
+  void WinMultiprocessParent() override {}
+
+  void WinMultiprocessChild() override {
+    HANDLE original_stdout = GetStdHandle(STD_OUTPUT_HANDLE);
+    HANDLE original_stderr = GetStdHandle(STD_ERROR_HANDLE);
+    SetStdHandle(STD_OUTPUT_HANDLE, original_stderr);
+
+    StartAndUseHandler();
+
+    SetStdHandle(STD_OUTPUT_HANDLE, original_stdout);
+  }
+};
+
+TEST(CrashpadClient, StartWithSameStdoutStderr) {
+  WinMultiprocess::Run<StartWithSameStdoutStderr>();
+}
+
+}  // namespace
+}  // namespace test
+}  // namespace crashpad