It is interesting that non duplicates have an access log like this:
"POST /index.php?main_page=checkout_confirmation
"POST /index.php?main_page=checkout_process
"GET /index.php?main_page=checkout_success&zenid=
"POST /ipn_main_handler.php HTTP/1.1" 200 - "-" "PayPal IPN

Whereas the duplicates are like this:
"POST /index.php?main_page=checkout_confirmation
"POST /index.php?main_page=checkout_process
"POST /ipn_main_handler.php HTTP/1.1" 200 (although the timestamp is actually 5 seconds after the next line item)
"POST /index.php?main_page=checkout_process
"GET /index.php?main_page=checkout_success&zenid=
"POST /ipn_main_handler.php HTTP/1.1" 200