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.

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'])