Enforce invoice number uniqueness.

oscar_invoices/abstract_models.py

@@ -71,7 +71,7 @@ class AbstractInvoice(models.Model):
         verbose_name=_('Legal Entity'))
 
     number = models.CharField(
-        _('Invoice number'), max_length=128)
+        _('Invoice number'), max_length=128, unique=True)
 
     order = models.OneToOneField(
         'order.Order', verbose_name=_('Order'), related_name='invoice',

oscar_invoices/migrations/0003_auto_20190403_1910.py

@@ -0,0 +1,18 @@
+# Generated by Django 2.1.8 on 2019-04-03 19:10
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+
+    dependencies = [
+        ('oscar_invoices', '0002_auto_20180822_1335'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='invoice',
+            name='number',
+            field=models.CharField(max_length=128, unique=True, verbose_name='Invoice number'),
+        ),
+    ]