Identifying Duplicate IPNs on subscr_signup

New Community Member

I am implementing a simple membership site and am testing my IPN code and I have run into what seems like a contradiction.
In the Paypal Developer Documentation ( a little ways down under IPN Protocol and Architecture is this:

Verify that the IPN is not a duplicate. To do this, save the transaction ID and last payment status in each IPN message in a database and verify that the current IPN's values for these fields are not already in this database.

Note: You can't rely on transaction ID alone to screen out duplicates, as this scenario shows: 1) PayPal sends you an IPN notifying you of a pending payment. 2) PayPal later sends you a second IPN telling you that the payment has completed. However, both IPNs contain the same transaction ID; therefore, if you were using just transaction ID to identify IPNs, you would to treat the "completed payment" IPN as a duplicate.

Now, when I look at what variables are passed upon a subscr_signup, neither the txn_id or payment_status fields are present, so how am I supposed to check that the signup is not a duplicate?

What am I missing here??

Many Thanks

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