Rob Blaauboer sent me this link to an article in De Telegraaf.nl. It suggest that Postbank does not check the name and number for each credit-payment transaction. That suggestion, to my knowledge, is incorrect but may be widespread. I have recently received an acknowledgement that a payment order (of a very small amount) could not be executed due to the fact that name and number of the receiver of the payment did not match.
Legally, anyone who receives a payment without any underlying legal basis (contract, gift etc) has a legal obligation to pay it back to the sender ('onverplichte betaling'). So even if one tries to keep hold of the money, the judge will rule that is has to be paid back.