Changed the update/insert to a postgres upsert to avoid concurrency issues.

tests/app/celery/test_reporting_tasks.py

@@ -438,7 +438,22 @@ def test_create_nightly_billing_update_when_record_exists(
 
     assert len(records) == 1
     assert records[0].bst_date == date(2018, 1, 14)
+    assert records[0].billable_units == 1
+
+    sample_notification(
+        notify_db,
+        notify_db_session,
+        created_at=datetime.now() - timedelta(days=1),
+        service=sample_service,
+        template=sample_template,
+        status='delivered',
+        sent_by=None,
+        international=False,
+        rate_multiplier=1.0,
+        billable_units=1,
+    )
 
     # run again, make sure create_nightly_billing() updates with no error
     create_nightly_billing()
     assert len(records) == 1
+    assert records[0].billable_units == 2