diff --git a/nova/cmd/manage.py b/nova/cmd/manage.py
index 38a3f33c6b06..f7f945b64ae3 100644
--- a/nova/cmd/manage.py
+++ b/nova/cmd/manage.py
@@ -932,8 +932,12 @@ class CellV2Commands(object):
         return transport_url
 
     def _non_unique_transport_url_database_connection_checker(self, ctxt,
-                                        transport_url, database_connection):
+                            cell_mapping, transport_url, database_connection):
         for cell in objects.CellMappingList.get_all(ctxt):
+            if cell_mapping and cell.uuid == cell_mapping.uuid:
+                # If we're looking for a specific cell, then don't check
+                # that one for same-ness to allow idempotent updates
+                continue
             if (cell.database_connection == database_connection or
                 cell.transport_url == transport_url):
                 print(_('The specified transport_url and/or '
@@ -1320,7 +1324,7 @@ class CellV2Commands(object):
                     'in the configuration file.'))
             return 1
         if (self._non_unique_transport_url_database_connection_checker(ctxt,
-            transport_url, database_connection)):
+            None, transport_url, database_connection)):
             return 2
         cell_mapping_uuid = uuidutils.generate_uuid()
         cell_mapping = objects.CellMapping(
@@ -1464,7 +1468,7 @@ class CellV2Commands(object):
         db_connection = db_connection or CONF.database.connection
 
         if (self._non_unique_transport_url_database_connection_checker(ctxt,
-            transport_url, db_connection)):
+                cell_mapping, transport_url, db_connection)):
                 # We use the return code 3 before 2 to avoid changing the
                 # semantic meanings of return codes.
                 return 3
diff --git a/nova/tests/unit/test_nova_manage.py b/nova/tests/unit/test_nova_manage.py
index 9ef27f965408..ad9d1836761c 100644
--- a/nova/tests/unit/test_nova_manage.py
+++ b/nova/tests/unit/test_nova_manage.py
@@ -1556,22 +1556,31 @@ class CellV2CommandsTestCase(test.NoDBTestCase):
 
     def test_non_unique_transport_url_database_connection_checker(self):
         ctxt = context.RequestContext()
-        objects.CellMapping(context=ctxt, uuid=uuidsentinel.cell1,
+        cell1 = objects.CellMapping(context=ctxt, uuid=uuidsentinel.cell1,
                             name='cell1',
                             transport_url='fake://mq1',
-                            database_connection='fake:///db1').create()
+                            database_connection='fake:///db1')
+        cell1.create()
         objects.CellMapping(context=ctxt, uuid=uuidsentinel.cell2,
                             name='cell2',
                             transport_url='fake://mq2',
                             database_connection='fake:///db2').create()
         resultf = self.commands.\
                     _non_unique_transport_url_database_connection_checker(
-                                        ctxt, 'fake://mq3', 'fake:///db3')
+                                        ctxt, None,
+                                        'fake://mq3', 'fake:///db3')
         resultt = self.commands.\
                     _non_unique_transport_url_database_connection_checker(
-                                        ctxt, 'fake://mq1', 'fake:///db1')
+                                        ctxt, None,
+                                        'fake://mq1', 'fake:///db1')
+        resultd = self.commands.\
+                    _non_unique_transport_url_database_connection_checker(
+                                        ctxt, cell1,
+                                        'fake://mq1', 'fake:///db1')
+
         self.assertFalse(resultf)
         self.assertTrue(resultt)
+        self.assertFalse(resultd)
         self.assertIn('exists', self.output.getvalue())
 
     def test_create_cell_use_params(self):