diff --git a/db/migrate/20151218232200_add_unique_index_to_category_users.rb b/db/migrate/20151218232200_add_unique_index_to_category_users.rb
new file mode 100644
index 00000000000..74cfe3f4f1a
--- /dev/null
+++ b/db/migrate/20151218232200_add_unique_index_to_category_users.rb
@@ -0,0 +1,21 @@
+class AddUniqueIndexToCategoryUsers < ActiveRecord::Migration
+  def up
+    execute <<SQL
+DELETE FROM category_users cu USING category_users cu1
+  WHERE cu.user_id = cu1.user_id AND
+        cu.category_id = cu1.category_id AND
+        cu.notification_level = cu1.notification_level AND
+        cu.id < cu1.id
+SQL
+
+    add_index :category_users, [:user_id, :category_id, :notification_level],
+        name: 'idx_category_users_u1', unique: true
+    add_index :category_users, [:category_id, :user_id, :notification_level],
+        name: 'idx_category_users_u2', unique: true
+  end
+
+  def down
+    remove_index :category_users, name: 'idx_category_users_u1'
+    remove_index :category_users, name: 'idx_category_users_u2'
+  end
+end