diff --git a/tools/edit_monitor/daemon_manager.py b/tools/edit_monitor/daemon_manager.py
index 36a7593..7d666fe 100644
--- a/tools/edit_monitor/daemon_manager.py
+++ b/tools/edit_monitor/daemon_manager.py
@@ -13,6 +13,8 @@
 # limitations under the License.
 
 
+import errno
+import fcntl
 import getpass
 import hashlib
 import logging
@@ -100,16 +102,32 @@
       logging.warning("Edit monitor for cog is not supported, exiting...")
       return
 
-    try:
-      self._stop_any_existing_instance()
-      self._write_pid_to_pidfile()
-      self._start_daemon_process()
-    except Exception as e:
-      logging.exception("Failed to start daemon manager with error %s", e)
-      self._send_error_event_to_clearcut(
-          edit_event_pb2.EditEvent.FAILED_TO_START_EDIT_MONITOR
-      )
-      raise e
+    setup_lock_file = pathlib.Path(tempfile.gettempdir()).joinpath(
+        self.pid_file_path.name + ".setup"
+    )
+    logging.info("setup lock file: %s", setup_lock_file)
+    with open(setup_lock_file, "w") as f:
+      try:
+        # Acquire an exclusive lock
+        fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+        self._stop_any_existing_instance()
+        self._write_pid_to_pidfile()
+        self._start_daemon_process()
+      except Exception as e:
+        if (
+            isinstance(e, IOError) and e.errno == errno.EAGAIN
+        ):  # Failed to acquire the file lock.
+          logging.warning("Another edit monitor is starting, exitinng...")
+          return
+        else:
+          logging.exception("Failed to start daemon manager with error %s", e)
+          self._send_error_event_to_clearcut(
+              edit_event_pb2.EditEvent.FAILED_TO_START_EDIT_MONITOR
+          )
+          raise e
+      finally:
+        # Release the lock
+        fcntl.flock(f, fcntl.LOCK_UN)
 
   def monitor_daemon(
       self,
@@ -414,18 +432,18 @@
     pids = []
 
     try:
-      output = subprocess.check_output(
-          ["ps", "-ef", "--no-headers"], text=True)
+      output = subprocess.check_output(["ps", "-ef", "--no-headers"], text=True)
       for line in output.splitlines():
-          parts = line.split()
-          process_path = parts[7]
-          if pathlib.Path(process_path).name == 'edit_monitor':
-            pid = int(parts[1])
-            if pid != self.pid:  # exclude the current process
-              pids.append(pid)
+        parts = line.split()
+        process_path = parts[7]
+        if pathlib.Path(process_path).name == "edit_monitor":
+          pid = int(parts[1])
+          if pid != self.pid:  # exclude the current process
+            pids.append(pid)
     except Exception:
       logging.exception(
-          "Failed to get pids of existing edit monitors from ps command.")
+          "Failed to get pids of existing edit monitors from ps command."
+      )
 
     return pids
 
diff --git a/tools/edit_monitor/daemon_manager_test.py b/tools/edit_monitor/daemon_manager_test.py
index e412ab0..be28965 100644
--- a/tools/edit_monitor/daemon_manager_test.py
+++ b/tools/edit_monitor/daemon_manager_test.py
@@ -14,6 +14,7 @@
 
 """Unittests for DaemonManager."""
 
+import fcntl
 import logging
 import multiprocessing
 import os
@@ -82,7 +83,8 @@
     # tests will be cleaned.
     tempfile.tempdir = self.working_dir.name
     self.patch = mock.patch.dict(
-        os.environ, {'ENABLE_ANDROID_EDIT_MONITOR': 'true'})
+        os.environ, {'ENABLE_ANDROID_EDIT_MONITOR': 'true'}
+    )
     self.patch.start()
 
   def tearDown(self):
@@ -102,6 +104,7 @@
     p = self._create_fake_deamon_process()
 
     self.assert_run_simple_daemon_success()
+    self.assert_no_subprocess_running()
 
   def test_start_success_with_existing_instance_already_dead(self):
     # Create a pidfile with pid that does not exist.
@@ -137,7 +140,9 @@
     # Verify no daemon process is started.
     self.assertIsNone(dm.daemon_process)
 
-  @mock.patch.dict(os.environ, {'ENABLE_ANDROID_EDIT_MONITOR': 'false'}, clear=True)
+  @mock.patch.dict(
+      os.environ, {'ENABLE_ANDROID_EDIT_MONITOR': 'false'}, clear=True
+  )
   def test_start_return_directly_if_disabled(self):
     dm = daemon_manager.DaemonManager(TEST_BINARY_FILE)
     dm.start()
@@ -154,6 +159,25 @@
     # Verify no daemon process is started.
     self.assertIsNone(dm.daemon_process)
 
+  def test_start_failed_other_instance_is_starting(self):
+    f = open(
+        pathlib.Path(self.working_dir.name).joinpath(
+            TEST_PID_FILE_PATH + '.setup'
+        ),
+        'w',
+    )
+    # Acquire an exclusive lock
+    fcntl.flock(f, fcntl.LOCK_EX | fcntl.LOCK_NB)
+
+    dm = daemon_manager.DaemonManager(TEST_BINARY_FILE)
+    dm.start()
+
+    # Release the lock
+    fcntl.flock(f, fcntl.LOCK_UN)
+    f.close()
+    # Verify no daemon process is started.
+    self.assertIsNone(dm.daemon_process)
+
   @mock.patch('os.kill')
   def test_start_failed_to_kill_existing_instance(self, mock_kill):
     mock_kill.side_effect = OSError('Unknown OSError')
@@ -177,6 +201,7 @@
         'edit_monitor'
     )
     pid_file_path_dir.mkdir(parents=True, exist_ok=True)
+
     # Makes the directory read-only so write pidfile will fail.
     os.chmod(pid_file_path_dir, 0o555)
 
@@ -216,7 +241,7 @@
         cclient=fake_cclient,
     )
     # set the fake total_memory_size
-    dm.total_memory_size = 100 * 1024 *1024
+    dm.total_memory_size = 100 * 1024 * 1024
     dm.start()
     dm.monitor_daemon(interval=1)
 
@@ -452,7 +477,7 @@
         pass
 
   def _create_fake_deamon_process(
-      self, name: str = ''
+      self, name: str = TEST_PID_FILE_PATH
   ) -> multiprocessing.Process:
     # Create a long running subprocess
     p = multiprocessing.Process(target=long_running_daemon)
@@ -463,7 +488,7 @@
         'edit_monitor'
     )
     pid_file_path_dir.mkdir(parents=True, exist_ok=True)
-    with open(pid_file_path_dir.joinpath(name + 'pid.lock'), 'w') as f:
+    with open(pid_file_path_dir.joinpath(name), 'w') as f:
       f.write(str(p.pid))
     return p