diff --git a/local_cname/cli.py b/local_cname/cli.py
index 8c4e60b..6094695 100644
--- a/local_cname/cli.py
+++ b/local_cname/cli.py
@@ -1,10 +1,12 @@
 import argparse
 import socket
+import sys
 import time
 import os
 from pathlib import Path
 
-from clickclick import Action, info
+from clickclick import Action, info, error
+from filelock import FileLock, Timeout
 
 
 def main():
@@ -14,11 +16,30 @@ def main():
     args = parser.parse_args()
 
     hosts_file = Path('/etc/hosts')
+    backup_file = hosts_file.with_suffix('.local-cname-backup')
+    lock_file = hosts_file.with_suffix('.local-cname-lock')
+
+    lock = FileLock(str(lock_file))
+    try:
+        with lock.acquire(timeout=1):
+            edit_etc_hosts(hosts_file, backup_file, args)
+    except Timeout:
+        error('Another instance of local-cname seems to be running.')
+        info('(if a previous process crashed, remove {} '
+             'and check the contents of {} and {})'.format(lock_file, hosts_file, backup_file))
+        sys.exit(1)
 
+
+def edit_etc_hosts(hosts_file, backup_file, args):
     with hosts_file.open() as fd:
         old_contents = fd.read()
 
-    backup_file = hosts_file.with_suffix('.local-cname-backup')
+    HEADER = '#### Start of entries generated by local-cname'
+    if HEADER in old_contents:
+        error('{} seems to have already been modified by local-cname.'.format(hosts_file))
+        info('Remove the local-cname header line from this file to proceed.')
+        sys.exit(1)
+
     with backup_file.open('w') as fd:
         fd.write(old_contents)
 
@@ -41,7 +62,7 @@ def main():
             with Action('Writing {} ..'.format(hosts_file)):
                 with hosts_file.open('w') as fd:
                     fd.write(old_contents)
-                    fd.write('#### Start of entries generated by local-cname\n')
+                    fd.write('{}\n'.format(HEADER))
                     for hostname, ip in entries:
                         fd.write('{} {}\n'.format(ip, hostname))
 
diff --git a/setup.py b/setup.py
index e39255a..606d7e3 100755
--- a/setup.py
+++ b/setup.py
@@ -22,7 +22,7 @@ def readme():
     keywords='dns hosts local',
     license='GNU General Public License v3 (GPLv3)',
     setup_requires=['flake8'],
-    install_requires=['clickclick'],
+    install_requires=['clickclick', 'filelock'],
     tests_require=[],
     classifiers=[
         'Programming Language :: Python',