Hi John,
Please I have just tested, and its confirmed that the Grouping key is the option to print cheques by invoice date, e.g. you might have grouping key configured in OBAP, and same grouping key is entered in vendor master data, please see below snapshots:
Grouping Key Configured in OBAP:
Grouping Key Assigned in Vendor Master Data:
The payment are paid by invoice date i.e. document date, and cheques are printed as per grouping key, i.e. invoice date/document date. This is the reason you are getting printed multiple cheques by document date.
Therefore, following two cheques are printed as per document dates, please see below snapshot. 
Please remove the grouping key from the vendor master data.
Note:
I hope in FBZP " Separate Payment for each Payment Reference" is also not marked
Vendor Master "Indv. Payment" check is also not marked.
I hope this will resolved your issue.
Please check and do let me know.
Thanks & Regards
Javed