diff --git a/kallithea/controllers/admin/settings.py b/kallithea/controllers/admin/settings.py
--- a/kallithea/controllers/admin/settings.py
+++ b/kallithea/controllers/admin/settings.py
@@ -351,7 +351,9 @@ class SettingsController(BaseController)
 
                 try:
                     ui_key = ui_key and ui_key.strip()
-                    if ui_value and ui_key:
+                    if ui_key in (x.ui_key for x in Ui.get_custom_hooks()):
+                        h.flash(_('Hook already exists'), category='error')
+                    elif ui_value and ui_key:
                         Ui.create_or_update_hook(ui_key, ui_value)
                         h.flash(_('Added new hook'), category='success')
                     elif hook_id:
diff --git a/kallithea/tests/functional/test_admin_settings.py b/kallithea/tests/functional/test_admin_settings.py
--- a/kallithea/tests/functional/test_admin_settings.py
+++ b/kallithea/tests/functional/test_admin_settings.py
@@ -56,6 +56,18 @@ class TestAdminSettingsController(TestCo
         response.mustcontain('test_hooks_1')
         response.mustcontain('new_value_of_hook_1')
 
+    def test_add_existing_custom_hook(self):
+        self.log_user()
+        response = self.app.post(url('admin_settings_hooks'),
+                                params=dict(new_hook_ui_key='test_hooks_1',
+                                            new_hook_ui_value='attempted_new_value',
+                                            _authentication_token=self.authentication_token()))
+
+        self.checkSessionFlash(response, 'Hook already exists')
+        response = response.follow()
+        response.mustcontain('test_hooks_1')
+        response.mustcontain('new_value_of_hook_1')
+
     def test_create_custom_hook_delete(self):
         self.log_user()
         response = self.app.post(url('admin_settings_hooks'),