Question

For subscr_payment and web_accept you have to check txn_id whether it has been sent to you before.

But how about

subscr_cancel, 
subscr_modify, 
subscr_eot, 
subscr_failed, 
subscr_signup

where there is no txn_id?

If I get subscr_modify 3 times from paypal how does my script know if the same IPN was sent 3 times or if the user modified their subscription 3 times?

Does anyone know if ipn_track_id remains the same if the same IPN message is sent multiple times?

Answer 1

If I get subscr_modify 3 times from paypal how does my script know if
the same IPN was sent 3 times or if the user modified their
subscription 3 times?

You are only going to get a repeated IPN response from PayPal if the first one wasn’t answered with a Http 200 Response Code, due to a Server 500 Error Code for example, or no answer.

But how about

subscr_cancel, subscr_modify, subscr_eot, subscr_failed,
subscr_signup

where there is no txn_id?

The PayPal documentation is really obscure and complicated, but I found a link that might be useful, it shows a list of every variable present in each type of IPN response:
https://www.paypalobjects.com/en_US/ebook/subscriptions/Appx-ipn_subscription_variables.html

subscr_id MIGHT be what you are looking for, but I’m not sure if it changes with every response or not.