PayPal IPN unique identifier

Question

I always assumed that txn_id sent with IPN message is unique. PayPal guidelines seem to support this idea - https://cms.paypal.com/us/cgi-bin/?cmd=_render-conte

Answer 1

IPN transactions are unique, also and transactions that change the payment will generate a new txn_id. For example refunds, so its a good idea to store all txn_id related to a single purchase. As of yet I don't know what transactions other than refunds generate a new txn_id, possibly reversals do too.