Teradata PT automatically deletes the checkpoint files of successful jobs, but retains the checkpoint files of unsuccessful jobs. If a previously run job is unsuccessful, you can either let the checkpoint files be reused by that failed job or delete them so they are not picked up by any other job executed under the same login name.
To avoid problems, you must either carefully maintain the checkpoint files of unsuccessful jobs, or, preferably, always specify a job name using the jobname option.