[1.10.x] Fixed #26881 -- Fixed duplicate managers in migrations.

When both parent and child models had managers with the same name and
a migrations opt-in both were added to the migration state.

Backport of dab83e5ba1 from master

django/db/migrations/state.py

@@ -446,12 +446,19 @@ class ModelState(object):
             bases = (models.Model,)
 
         managers = []
+        manager_names = set()
         default_manager_shim = None
         for manager in model._meta.managers:
-            if manager.use_in_migrations:
+            manager_name = force_text(manager.name)
+            if manager_name in manager_names:
+                # Skip overridden managers.
+                continue
+            elif manager.use_in_migrations:
+                # Copy managers usable in migrations.
                 new_manager = copy.copy(manager)
                 new_manager._set_creation_counter()
             elif manager is model._base_manager or manager is model._default_manager:
+                # Shim custom managers used as default and base managers.
                 new_manager = models.Manager()
                 new_manager.model = manager.model
                 new_manager.name = manager.name
@@ -459,7 +466,8 @@ class ModelState(object):
                     default_manager_shim = new_manager
             else:
                 continue
-            managers.append((force_text(manager.name), new_manager))
+            manager_names.add(manager_name)
+            managers.append((manager_name, new_manager))
 
         # Ignore a shimmed default manager called objects if it's the only one.
         if managers == [('objects', default_manager_shim)]:

tests/migrations/test_state.py

@@ -210,6 +210,37 @@ class StateTests(SimpleTestCase):
         author_state = project_state.models['migrations', 'author']
         self.assertEqual(author_state.managers, [])
 
+    def test_no_duplicate_managers(self):
+        """
+        When a manager is added with `use_in_migrations = True` and a parent
+        model had a manager with the same name and `use_in_migrations = True`,
+        the parent's manager shouldn't appear in the model state (#26881).
+        """
+        new_apps = Apps(['migrations'])
+
+        class PersonManager(models.Manager):
+            use_in_migrations = True
+
+        class Person(models.Model):
+            objects = PersonManager()
+
+            class Meta:
+                abstract = True
+
+        class BossManager(PersonManager):
+            use_in_migrations = True
+
+        class Boss(Person):
+            objects = BossManager()
+
+            class Meta:
+                app_label = 'migrations'
+                apps = new_apps
+
+        project_state = ProjectState.from_apps(new_apps)
+        boss_state = project_state.models['migrations', 'boss']
+        self.assertEqual(boss_state.managers, [('objects', Boss.objects)])
+
     def test_custom_default_manager(self):
         new_apps = Apps(['migrations'])