Useful topic to cover but isn't the final solution over-complex?

Couldn't you just use an inequality of PaymentID's on a self-join as follows..?

DELETE Payment

WHERE PaymentID IN

(SELECT P1.PaymentID

FROM Payment P1

INNER JOIN Payment P2

ON P1.CustomerNumber = p2.CustomerNumber

AND

p1.PostedDatetime = p2.PostedDatetime

AND

p1.PaymentAmt = p2.PaymentAmt

AND

p1.PaymentID > p2.PaymentID)

Mark