What this message is telling us is that the code is trying to save an invalid duel. An invalid duel would be, among other things, Fleshnblood_78 vs Fleshnblood_78.
The problem is I don't know what code is doing this. So, you'll have to help me out. Next time this happens, note the Duel Type (RPS, Rock Paper Scossirs, Toggle), the challenger, when (relative to when you post the message), and the choices. That way i will be able to track it down further.