Web payments with Paypal may record double payments.
This is a timing issue with the aeon.dll so it can't be easily reproduced.
When a payment has been completed with PayPal, it will send both a PDT response (an immediate response that redirects the user back to your page with payment confirmation details) and an IPN response (which is sent directly from PayPal to your server and can ensure you're notified of a successful payment even if the customer immediately closes their browser after completing payment on PayPal's site). The order and timing of these notifications is not guaranteed.
Released in Aeon 4.0