fix(backend): Make task registering more robust (#9586)

* Make task registering more robust
See https://github.com/inventree/InvenTree/issues/9579

* add unit test

src/backend/InvenTree/plugin/base/integration/ScheduleMixin.py

@@ -180,7 +180,15 @@ class ScheduleMixin:
                     obj['func'] = 'plugin.registry.call_plugin_function'
                     obj['args'] = f"'{slug}', '{func_name}'"
 
-                if Schedule.objects.filter(name=task_name).exists():
+                tasks = Schedule.objects.filter(name=task_name)
+                if len(tasks) > 1:
+                    logger.info(
+                        "Found multiple tasks; Adding a new scheduled task '%s'",
+                        task_name,
+                    )
+                    tasks.delete()
+                    Schedule.objects.create(**obj)
+                elif len(tasks) == 1:
                     # Scheduled task already exists - update it!
                     logger.info("Updating scheduled task '%s'", task_name)
                     instance = Schedule.objects.get(name=task_name)

src/backend/InvenTree/plugin/samples/integration/test_scheduled_task.py

@@ -2,10 +2,10 @@
 
 from django.test import TestCase
 
-from plugin import InvenTreePlugin, registry
+from plugin import InvenTreePlugin
 from plugin.helpers import MixinImplementationError
 from plugin.mixins import ScheduleMixin
-from plugin.registry import call_plugin_function
+from plugin.registry import call_plugin_function, registry
 
 
 class ExampleScheduledTaskPluginTests(TestCase):
@@ -42,9 +42,19 @@ class ExampleScheduledTaskPluginTests(TestCase):
         # test updating the schedule
         hello_schedule = Schedule.objects.get(name='plugin.schedule.hello')
         self.assertEqual(hello_schedule.minutes, 45)
-        # change the schedule and reregister
+        # change the schedule and reregister -> the interval should be preserved
         plg.scheduled_tasks['hello']['minutes'] = 15
+        # add a doubly scheduled task - this should be removed
+        Schedule.objects.create(name='plugin.schedule.hello')
+        self.assertEqual(
+            Schedule.objects.filter(name='plugin.schedule.hello').count(), 2
+        )
+
         plg.register_tasks()
+        # The duplicate task should be removed
+        self.assertEqual(
+            Schedule.objects.filter(name='plugin.schedule.hello').count(), 1
+        )
 
         # Check that the schedule was updated
         hello_schedule = Schedule.objects.get(name='plugin.schedule.hello')