Fixed #19327 -- Added handling of double login attempts in admin.

Thanks to Krzysztof Jurewicz for initial patch and
adupin for tests.

AUTHORS

@@ -606,6 +606,7 @@ answer newbie questions, and generally made Django that much better:
     Jarek Zgoda <jarek.zgoda@gmail.com>
     Cheng Zhang
     Hannes Struß <x@hannesstruss.de>
+    Deric Crago <deric.crago@gmail.com>
 
 A big THANK YOU goes to:
 

django/contrib/admin/sites.py

@@ -2,7 +2,7 @@ from functools import update_wrapper
 from django.http import Http404, HttpResponseRedirect
 from django.contrib.admin import ModelAdmin, actions
 from django.contrib.admin.forms import AdminAuthenticationForm
-from django.contrib.auth import REDIRECT_FIELD_NAME
+from django.contrib.auth import logout as auth_logout, REDIRECT_FIELD_NAME
 from django.contrib.contenttypes import views as contenttype_views
 from django.views.decorators.csrf import csrf_protect
 from django.db.models.base import ModelBase
@@ -193,6 +193,8 @@ class AdminSite(object):
         cacheable=True.
         """
         def inner(request, *args, **kwargs):
+            if LOGIN_FORM_KEY in request.POST and request.user.is_authenticated():
+                auth_logout(request)
             if not self.has_permission(request):
                 if request.path == reverse('admin:logout',
                                            current_app=self.name):

tests/admin_views/tests.py

@@ -6,7 +6,7 @@ import re
 import datetime
 try:
     from urllib.parse import urljoin
-except ImportError:     # Python 2
+except ImportError:  # Python 2
     from urlparse import urljoin
 
 from django.conf import settings, global_settings
@@ -981,6 +981,32 @@ class AdminViewPermissionsTest(TestCase):
         login = self.client.post('/test_admin/admin/', dict(self.super_login, **new_next), QUERY_STRING=query_string)
         self.assertRedirects(login, redirect_url)
 
+    def testDoubleLoginIsNotAllowed(self):
+        """Regression test for #19327"""
+        response = self.client.get('/test_admin/admin/')
+        self.assertEqual(response.status_code, 200)
+
+        # Establish a valid admin session
+        login = self.client.post('/test_admin/admin/', self.super_login)
+        self.assertRedirects(login, '/test_admin/admin/')
+        self.assertFalse(login.context)
+
+        # Logging in with non-admin user fails
+        login = self.client.post('/test_admin/admin/', self.joepublic_login)
+        self.assertEqual(login.status_code, 200)
+        self.assertContains(login, ERROR_MESSAGE)
+
+        # Establish a valid admin session
+        login = self.client.post('/test_admin/admin/', self.super_login)
+        self.assertRedirects(login, '/test_admin/admin/')
+        self.assertFalse(login.context)
+
+        # Logging in with admin user while already logged in
+        login = self.client.post('/test_admin/admin/', self.super_login)
+        self.assertRedirects(login, '/test_admin/admin/')
+        self.assertFalse(login.context)
+        self.client.get('/test_admin/admin/logout/')
+
     def testAddView(self):
         """Test add view restricts access and actually adds items."""
 
@@ -2547,7 +2573,7 @@ class AdminCustomQuerysetTest(TestCase):
                 self.assertNotContains(response, 'Primary key = %s' % i)
 
     def test_changelist_view_count_queries(self):
-        #create 2 Person objects
+        # create 2 Person objects
         Person.objects.create(name='person1', gender=1)
         Person.objects.create(name='person2', gender=2)