When searching for human tasks to determine if the current user can complete it, filter on the "completed" flag.

Front-end -- enable the form if you receive an onUnathorized error because the thing you are unauthorized to do might have nothing to do with whether you can submit the form.

src/spiffworkflow_backend/services/authorization_service.py

@@ -457,6 +457,7 @@ class AuthorizationService:
         human_task = HumanTaskModel.query.filter_by(
             task_name=spiff_task.task_spec.name,
             process_instance_id=process_instance_id,
+            completed=False
         ).first()
         if human_task is None:
             raise HumanTaskNotFoundError(