diff --git a/django_db_constraints/autodetector.py b/django_db_constraints/autodetector.py
index a21d409..3628de1 100644
--- a/django_db_constraints/autodetector.py
+++ b/django_db_constraints/autodetector.py
@@ -1,9 +1,9 @@
 from django.db.migrations import operations
 from django.db.migrations.autodetector import MigrationAutodetector
 
+from builtins import super
 from .operations import AlterConstraints
 
-
 class MigrationAutodetectorWithDbConstraints(MigrationAutodetector):
     db_constraints_operations = []
 
diff --git a/django_db_constraints/operations.py b/django_db_constraints/operations.py
index 933cef1..d137c98 100644
--- a/django_db_constraints/operations.py
+++ b/django_db_constraints/operations.py
@@ -1,3 +1,4 @@
+from builtins import super
 from django.db.migrations.operations.models import ModelOptionOperation
 
 
@@ -24,8 +25,8 @@ def database_forwards(self, app_label, schema_editor, from_state, to_state):
         if self.allow_migrate_model(schema_editor.connection.alias, to_model):
             from_model = from_state.apps.get_model(app_label, self.name)
 
-            to_constraints = getattr(to_model._meta, self.option_name, {}).keys()
-            from_constraints = getattr(from_model._meta, self.option_name, {}).keys()
+            to_constraints = set(getattr(to_model._meta, self.option_name, {}).keys())
+            from_constraints = set(getattr(from_model._meta, self.option_name, {}).keys())
 
             table_operations = tuple(
                 'DROP CONSTRAINT IF EXISTS {name}'.format(
diff --git a/setup.py b/setup.py
index ff3bfb8..5fb3b28 100644
--- a/setup.py
+++ b/setup.py
@@ -6,7 +6,7 @@ def readme():
 
 setup(
     name='django-db-constraints',
-    version='0.3.0',
+    version='0.3.0a',
     author='shangxiao',
     description='Add database table-level constraints to your Django model\'s Meta',
     long_description=readme(),