Fixed #27612 -- Added a check for duplicate URL instance namespaces.

django/core/checks/urls.py

 from __future__ import unicode_literals
 
+from collections import Counter
+
 from django.conf import settings
 from django.utils import six
 
-from . import Error, Tags, register
+from . import Error, Tags, Warning, register
 
 
 @register(Tags.urls)
@@ -28,6 +30,43 @@ def check_resolver(resolver):
         return []
 
 
+@register(Tags.urls)
+def check_url_namespaces_unique(app_configs, **kwargs):
+    """
+    Warn if URL namespaces used in applications aren't unique.
+    """
+    if not getattr(settings, 'ROOT_URLCONF', None):
+        return []
+
+    from django.urls import get_resolver
+    resolver = get_resolver()
+    all_namespaces = _load_all_namespaces(resolver)
+    counter = Counter(all_namespaces)
+    non_unique_namespaces = [n for n, count in counter.items() if count > 1]
+    errors = []
+    for namespace in non_unique_namespaces:
+        errors.append(Warning(
+            "URL namespace '{}' isn't unique. You may not be able to reverse "
+            "all URLs in this namespace".format(namespace),
+            id="urls.W005",
+        ))
+    return errors
+
+
+def _load_all_namespaces(resolver):
+    """
+    Recursively load all namespaces from URL patterns.
+    """
+    url_patterns = getattr(resolver, 'url_patterns', [])
+    namespaces = [
+        url.namespace for url in url_patterns
+        if getattr(url, 'namespace', None) is not None
+    ]
+    for pattern in url_patterns:
+        namespaces.extend(_load_all_namespaces(pattern))
+    return namespaces
+
+
 def get_warning_for_invalid_pattern(pattern):
     """
     Return a list containing a warning that the pattern is invalid.

docs/ref/checks.txt

@@ -673,3 +673,5 @@ The following checks are performed on your URL configuration:
   references.
 * **urls.E004**: Your URL pattern ``<pattern>`` is invalid. Ensure that
   ``urlpatterns`` is a list of :func:`~django.conf.urls.url()` instances.
+* **urls.W005**: URL namespace ``<namespace>`` isn't unique. You may not be
+  able to reverse all URLs in this namespace.

tests/check_framework/test_urls.py

 from django.conf import settings
+from django.core.checks.messages import Warning
 from django.core.checks.urls import (
-    check_url_config, get_warning_for_invalid_pattern,
+    check_url_config, check_url_namespaces_unique, get_warning_for_invalid_pattern,
 )
 from django.test import SimpleTestCase
 from django.test.utils import override_settings
@@ -94,3 +95,23 @@ class CheckUrlsTest(SimpleTestCase):
     def test_get_warning_for_invalid_pattern_other(self):
         warning = get_warning_for_invalid_pattern(object())[0]
         self.assertIsNone(warning.hint)
+
+    @override_settings(ROOT_URLCONF='check_framework.urls.non_unique_namespaces')
+    def test_check_non_unique_namespaces(self):
+        result = check_url_namespaces_unique(None)
+        self.assertEqual(len(result), 2)
+        non_unique_namespaces = ['app-ns1', 'app-1']
+        warning_messages = [
+            "URL namespace '{}' isn't unique. You may not be able to reverse "
+            "all URLs in this namespace".format(namespace)
+            for namespace in non_unique_namespaces
+        ]
+        for warning in result:
+            self.assertIsInstance(warning, Warning)
+            self.assertEqual('urls.W005', warning.id)
+            self.assertIn(warning.msg, warning_messages)
+
+    @override_settings(ROOT_URLCONF='check_framework.urls.unique_namespaces')
+    def test_check_unique_namespaces(self):
+        result = check_url_namespaces_unique(None)
+        self.assertEqual(result, [])

tests/check_framework/urls/non_unique_namespaces.py

+from django.conf.urls import include, url
+
+common_url_patterns = ([
+    url(r'^app-ns1/', include([])),
+    url(r'^app-url/', include([])),
+], 'app-ns1')
+
+urlpatterns = [
+    url(r'^app-ns1-0/', include(common_url_patterns)),
+    url(r'^app-ns1-1/', include(common_url_patterns)),
+    url(r'^app-some-url/', include(([], 'app'), namespace='app-1')),
+    url(r'^app-some-url-2/', include(([], 'app'), namespace='app-1'))
+]

tests/check_framework/urls/unique_namespaces.py

+from django.conf.urls import include, url
+
+common_url_patterns = ([
+    url(r'^app-ns1/', include([])),
+    url(r'^app-url/', include([])),
+], 'common')
+
+urlpatterns = [
+    url(r'^app-ns1-0/', include(common_url_patterns, namespace='app-include-1')),
+    url(r'^app-ns1-1/', include(common_url_patterns, namespace='app-include-2'))
+]