fix(crons): Fix backfill to handle multiple objects returned (#98268)

There shouldn't be multiple objects here, but there's no unique
constraint and so they can happen during a race condition. If these rows
exist at all we have nothing to do, so just skip

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Author: wedamija

src/sentry/monitors/migrations/0009_backfill_monitor_detectors.py

@@ -30,11 +30,16 @@ def backfill_monitor_detectors(apps: StateApps, schema_editor: BaseDatabaseSchem
 
         # Copied from `ensure_cron_detector`
         with transaction.atomic(router.db_for_write(DataSource)):
-            data_source, created = DataSource.objects.get_or_create(
-                type=DATA_SOURCE_CRON_MONITOR,
-                organization_id=monitor.organization_id,
-                source_id=str(monitor.id),
-            )
+            try:
+                data_source, created = DataSource.objects.get_or_create(
+                    type=DATA_SOURCE_CRON_MONITOR,
+                    organization_id=monitor.organization_id,
+                    source_id=str(monitor.id),
+                )
+            except DataSource.MultipleObjectsReturned:
+                # If these rows already exist just skip. We shouldn't have multiple rows, but because there's no unique
+                # key on `type, organization_id, source_id` we can have race conditions.
+                continue
             if created:
                 detector = Detector.objects.create(
                     # MonitorIncidentType.slug