...are not top-notch, apparently. Spent the last two and a half hours trying to figure out what was wrong with a message.
Turns out, nothing. It was the addressing that was messed up. The trading partner used the service proxy's address as the service endpoint. Resulting in the service proxy routing the message to itself, declaring the message a duplicate, reporting that error back to what it thought was a client (and was actually itself), and being quite startled at the response it got back from what it thought was a third party trading partner (and was actually itself) declaring that the message it had just sent was a duplicate.
At least we had the proxy checking tokens for duplicates. An infinite loop in our request gateway is a pretty big DOS risk.