Sara-thank you for looking into this issue for us.
Here is a brief summary of what occured.
UBS and Enron entered into a Sterling-UBS spot transaction on 11-19-01, which both parties confirmed and settled on 11-21.
(UBS pays USD 7,083,500 and receives 5 mio sterling).
On 11-21 UBS and Enron erroneously confirmed the same trade again for settlement on 11-21.
UBS made a second USD 7,083,500 payment to Enron's account at BA-Dallas on 11-21 (payment reference: FED IMA 1121B1Q8052C001081).
UBS did not receive the Sterling it was expecting and for the next few days Enron and UBS tried to resolve the non-payment based on a mistaken assumption that Enron owed 5 mio Sterling to UBS.
On 11-28, UBS realized that the trade was duped and on 11-29 informed Enron of the nature of the error and requested that the USD payment be returned.
Enron agreed to return the USD payment on 11-29.
The returned payment was never received.

UBS FX payment