Mr. Richard Walsh:

Duplicate payments could be the website or the merchant's site accidentally sending through two payments. Every payment has a unique identifier and the banks check to see whether a unique identifier has not been used before. If two transactions came in that were absolutely identical it would raise a red flag. Of course it is possible that if something is being bought online the website might have reprocessed a payment with a different identity number. This may not necessarily be deliberate or fraudulent. It could be an accident. They would be seen as two separate payments, albeit coming from the one website.