7👍
I have figured this one out. The short answer is to make sure your receiver functions are working correctly.
When Paypal sends an IPN to the URL you have specified, they are expecting a response with HTTP status code 200. If the response code is anything else, they will retry. Even if you process a callback and get a VERIFIED message, the IPN from Paypal needs a response of 200 OK.
I had a look through my Apache access logs and discovered that when I had errors in my receiver function for the payment_was_successful signal, the initial IPN from paypal received a HTTP status code 500.
The django-paypal package responds with HttpResponse("OKAY") only after everything else has been processed, including the postback to Paypal which was returning “VERIFIED”, the PayPalIPN object being saved to the database, and the sending of signals. When things went wrong in my signal receiver function and an unhandled exception was raised, Django was responding with a HTTP status code 500.
When Paypal retried the IPN, the django-paypal package was detecting a duplicate txn_id and sending the payment_was_flagged signal. My receiver function for this signal was error free, so Paypal received the HTTP status code 200 it was expecting and stopped retrying.
Hopefully this helps someone else out there in the future.