sandbox: allow running make build_sandbox multiple times

Makefile

 	sites/sandbox/manage.py loaddata sites/sandbox/fixtures/child_products.json
 	sites/sandbox/manage.py oscar_import_catalogue sites/sandbox/fixtures/*.csv
 	sites/sandbox/manage.py oscar_import_catalogue_images sites/sandbox/fixtures/images.tar.gz
-	sites/sandbox/manage.py oscar_populate_countries
+	sites/sandbox/manage.py oscar_populate_countries --initial-only
 	sites/sandbox/manage.py loaddata sites/_fixtures/pages.json sites/_fixtures/auth.json sites/_fixtures/ranges.json sites/_fixtures/offers.json
 	sites/sandbox/manage.py loaddata sites/sandbox/fixtures/orders.json
 	sites/sandbox/manage.py clear_index --noinput

src/oscar/apps/partner/importers.py

 
         # Category
         cat = create_from_breadcrumbs(category_str)
-        ProductCategory.objects.create(product=item, category=cat)
+        ProductCategory.objects.update_or_create(product=item, category=cat)
 
         return item