Refs #26521 -- Adjusted CreateModel bases validation to account for mixins.

Thanks Collin for the report.

django/db/migrations/operations/models.py

@@ -56,12 +56,11 @@ class CreateModel(ModelOperation):
         # Sanity-check that there are no duplicated field names, bases, or
         # manager names
         _check_for_duplicates('fields', (name for name, _ in self.fields))
-        _check_for_duplicates(
+        _check_for_duplicates('bases', (
-            'bases',
+            base._meta.label_lower if hasattr(base, '_meta') else
-            (base._meta.label_lower if isinstance(base, models.base.ModelBase) else base.lower()
+            base.lower() if isinstance(base, six.string_types) else base
             for base in self.bases
-             if base is not models.Model)
+        ))
-        )
         _check_for_duplicates('managers', (name for name, _ in self.managers))
 
     def deconstruct(self):

tests/migrations/test_operations.py

@@ -19,6 +19,10 @@ except ImportError:
     sqlparse = None
 
 
+class Mixin(object):
+    pass
+
+
 class OperationTestBase(MigrationTestBase):
     """
     Common functions to help test operations.
@@ -244,6 +248,20 @@ class OperationTests(OperationTestBase):
                 fields=[],
                 bases=(UnicodeModel, 'migrations.UnicodeModel',),
             )
+        message = "Found duplicate value <class 'django.db.models.base.Model'> in CreateModel bases argument."
+        with self.assertRaisesMessage(ValueError, message):
+            migrations.CreateModel(
+                "Pony",
+                fields=[],
+                bases=(models.Model, models.Model,),
+            )
+        message = "Found duplicate value <class 'migrations.test_operations.Mixin'> in CreateModel bases argument."
+        with self.assertRaisesMessage(ValueError, message):
+            migrations.CreateModel(
+                "Pony",
+                fields=[],
+                bases=(Mixin, Mixin,),
+            )
 
     def test_create_model_with_duplicate_manager_name(self):
         with self.assertRaisesMessage(ValueError, 'Found duplicate value objects in CreateModel managers argument.'):