Thank you very much. I read someplace if you run pg_start_backup twice the backup.old will be created, but there was not much beyond that and now I can't seem to find the reference.
Scanning the docs and logic tells me that attempting to do pg_start_backup twice in a row should result in the second attempt giving an error...but I could be misinformed.
The file pg_start_backup creates is named "backup_label" and so I'd also expect any attempt to add an old suffix would keep the same base name...