Added a simple check in JobExecutor.submit() to test if a job's

executor property is already set (which would indicate that it was
submitted before).

Author: hategan

src/psij/job_executor.py

@@ -105,6 +105,8 @@ def _check_job(self, job: Job) -> JobSpec:
                         raise TypeError('environment key "%s" has non-string value (%s)'
                                         % (k, type(v).__name__))
 
+        if job.executor is not None:
+            raise InvalidJobException('Job is already associated with an executor')
         job.executor = self
         return spec