API: Fix validation of duplicate customer email addresses

src/pretix/api/serializers/organizer.py

@@ -94,6 +94,14 @@ class CustomerSerializer(I18nAwareModelSerializer):
             data['name_parts']['_scheme'] = self.context['request'].organizer.settings.name_scheme
         return data
 
+    def validate_email(self, value):
+        qs = Customer.objects.filter(organizer=self.context['organizer'], email__iexact=value)
+        if self.instance and self.instance.pk:
+            qs = qs.exclude(pk=self.instance.pk)
+        if qs.exists():
+            raise ValidationError(_("An account with this email address is already registered."))
+        return value
+
 
 class CustomerCreateSerializer(CustomerSerializer):
     send_email = serializers.BooleanField(default=False, required=False, allow_null=True)

src/tests/api/test_customers.py

@@ -104,6 +104,34 @@ def test_customer_create(token_client, organizer):
     assert len(djmail.outbox) == 0
 
 
+@pytest.mark.django_db
+def test_customer_create_email_unique(token_client, organizer):
+    resp = token_client.post(
+        '/api/v1/organizers/{}/customers/'.format(organizer.slug),
+        format='json',
+        data={
+            'identifier': 'IGNORED',
+            'email': 'bar@example.com',
+            'password': 'foobar',
+            'is_active': True,
+            'is_verified': True,
+        }
+    )
+    assert resp.status_code == 201
+    resp = token_client.post(
+        '/api/v1/organizers/{}/customers/'.format(organizer.slug),
+        format='json',
+        data={
+            'identifier': 'IGNORED',
+            'email': 'bar@example.com',
+            'password': 'foobar',
+            'is_active': True,
+            'is_verified': True,
+        }
+    )
+    assert resp.status_code == 400
+
+
 @pytest.mark.django_db
 def test_customer_create_send_email(token_client, organizer):
     resp = token_client.post(