Signer does more than one axfr into same file, filling disk

When ods is started for the first time it works ok when fetching a large zone, but if it receives notifies when doing an axfr, it seems to start another axfr into the same file. This would eventually fill up the disk.
Advised by Sion to set Workers to 1, and since then we have had a further issue, this time when doing a ods-control stop and start. At this point it started doing the same thing - what appears to be multiple axfr transfers into the same file in the working directory.