diff --git a/CHANGES.rst b/CHANGES.rst
index 591d870..0079324 100644
--- a/CHANGES.rst
+++ b/CHANGES.rst
@@ -6,7 +6,7 @@ CHANGES
 
 - New option ``doctests`` to run Pyflakes checks on doctests too
 - New option ``jobs`` to launch multiple jobs in parallel
-- Fix Git and Mercurial hooks, issues #88, #133 and #148
+- Fix Git and Mercurial hooks: issues #88, #133, #148 and #149
 - Fix crashes with Python 3.4 by upgrading dependencies
 - Fix traceback when running tests with Python 2.6
 - Fix the setuptools command ``python setup.py flake8`` to read
diff --git a/flake8/hooks.py b/flake8/hooks.py
index 289b66f..9542279 100644
--- a/flake8/hooks.py
+++ b/flake8/hooks.py
@@ -228,6 +228,8 @@ def install_hook():
 
     status = 0
     if 'git' in vcs:
+        if os.path.exists(vcs):
+            sys.exit('Error: hook already exists (%s)' % vcs)
         with open(vcs, 'w') as fd:
             fd.write(git_hook_file)
         # rwxr--r--