I've set it exactly as needed, but it seems I'm still getting duplicate orders and the error logs are still generating.
It seems to be displaying two paypal transactions per order, one through PDK and one through IPN.
The customer is only charged once, but 2 orders are generating.
I really hope there is some more troubleshooting that can be done to help resolve this before I am forced to stop using the PayPal Website Payments Standard!

This info is in the zen_paypal database for the same order number:

`module_name`, `module_mode`, `reason_code`, `payment_type`,`payment_status`, `payment_date` ......`date_added`, `memo`
'paypal 1.5.7', 'PDT', '', 'instant', 'Completed', - 2021-09-02 00:10:39 ....... '2021-09-02 17:10:54', '{Successful PDT Confirmation - Record auto-generated by payment module}
'paypal (ipn-handler)', 'IPN', '', 'instant', 'Completed' - 2021-09-02 00:10:39 ....... '2021-09-02 17:11:01', '{Record generated by IPN} [ProtectionEligibility:Eligible]