Fixed #12288 -- Added unique validation for INSTALLED_APPS

2 files changed, 24 insertions, 0 deletions

diff --git a/django/conf/__init__.py b/django/conf/__init__.py
index 56d9ab21c9..e628af748c 100644
--- a/django/conf/__init__.py
+++ b/django/conf/__init__.py
@@ -107,6 +107,9 @@ class BaseSettings(object):
         elif name == "ALLOWED_INCLUDE_ROOTS" and isinstance(value, six.string_types):
             raise ValueError("The ALLOWED_INCLUDE_ROOTS setting must be set "
                 "to a tuple, not a string.")
+        elif name == "INSTALLED_APPS" and len(value) != len(set(value)):
+            raise ImproperlyConfigured("The INSTALLED_APPS setting must contain unique values.")
+
         object.__setattr__(self, name, value)
 
 
diff --git a/tests/settings_tests/tests.py b/tests/settings_tests/tests.py
index 3673031447..4031d09a58 100644
--- a/tests/settings_tests/tests.py
+++ b/tests/settings_tests/tests.py
@@ -226,6 +226,27 @@ class TestComplexSettingOverride(TestCase):
             self.assertEqual('Overriding setting TEST_WARN can lead to unexpected behaviour.', str(w[-1].message))
 
 
+class UniqueSettngsTests(TestCase):
+    """
+    Tests for the INSTALLED_APPS setting.
+    """
+    settings_module = settings
+
+    def setUp(self):
+        self._installed_apps = self.settings_module.INSTALLED_APPS
+
+    def tearDown(self):
+        self.settings_module.INSTALLED_APPS = self._installed_apps
+
+    def test_unique(self):
+        """
+        An ImproperlyConfigured exception is raised if the INSTALLED_APPS contains
+        any duplicate strings.
+        """
+        with self.assertRaises(ImproperlyConfigured):
+            self.settings_module.INSTALLED_APPS = ("myApp1", "myApp1", "myApp2", "myApp3")
+
+
 class TrailingSlashURLTests(TestCase):
     """
     Tests for the MEDIA_URL and STATIC_URL settings.