They're different APIs and believe different purchases at PayPal so it'd depend entirely which you bought access to. Check the below within your PayPal account as it seams your account is not setup to block duplicate transactions, which would at least stop the double billing.
www.paypal.com/us/smarthelp/article/how-do-i-avoid-duplicate-transactions-ts1097
Looks like even if INVNUM is supplied if that setting isn't enabled PayPal won't block the duplicate transactions. We still need to investigate and fix the double payment attempt on our end, but for now that'll resolve your issue.