had an issue where our clients payment gateway would duplicate the charge (at the gateway...not at the application) before sending it to the bank officially - the bank would detect the duplication then void both charges.

the gateway service admitted this was a bug, the bank it was tied to admitted it was a bug - but they wouldnt fix it. so my solution was to send a special uid with the original transaction (put it in a special field) and had the bank track that one as the "known good"

the funny thing? next version of the gateway api included this as a feature, but i got no credit.