Fixed #26521 -- Validated CreateModel bases, fields and managers for duplicates.

django/db/migrations/operations/models.py

@@ -12,6 +12,16 @@ from .fields import (
 )
 
 
+def _check_for_duplicates(arg_name, objs):
+    used_vals = set()
+    for val in objs:
+        if val in used_vals:
+            raise ValueError(
+                'Found duplicate %s in CreateModel operation.' % arg_name
+            )
+        used_vals.add(val)
+
+
 class ModelOperation(Operation):
     def __init__(self, name):
         self.name = name
@@ -43,6 +53,16 @@ class CreateModel(ModelOperation):
         self.bases = bases or (models.Model,)
         self.managers = managers or []
         super(CreateModel, self).__init__(name)
+        # Sanity-check that there are no duplicated field names, bases, or
+        # manager names
+        _check_for_duplicates("field", (name for name, _ in self.fields))
+        _check_for_duplicates(
+            "base",
+            (base._meta.label_lower if isinstance(base, models.base.ModelBase) else base.lower()
+             for base in self.bases
+             if base is not models.Model)
+        )
+        _check_for_duplicates("manager", (name for name, _ in self.managers))
 
     def deconstruct(self):
         kwargs = {