A few things I have found:
1. For the duel_rounds variable,
There is no case in event, duel.1, for when duel_rounds = 3.
This can lead to duels ending/not starting, inexplicably.
2. Not all duel ending events reset a characters duel_rounds variable to 0, some do, some don't(don't know if this really matters much mechanically, or if it only effects the duel flavor events).
3. The duel event in which you kill your opponent outright, without a wounding, or yield chance, does not set the flag, flag_killed_opponent, and so does not fire a duel_output event.
This is because no IF statement in duel_output checks for Only having the battlefield_duel flag(which is the case when you kill opponent in battlefield duel in this way.)
This also means that the battlefield_duel flag is not cleared from the character.
Again, I don't think this one actually effects anything other than the flavor events.
Edit:
After testing some things with variables,
it looks like in the case of variables that:
NOT = { check_variable = { which = duel_rounds value = 0 } }
means, equal to, OR less than, 0
And that:
check_variable = { which = duel_rounds value = 0 }
means, greater than 0 (equal to 0 in this case is False!)

hmy:
If this is the case, then duel.1 is actually missing a case for when duel_rounds = 4, not 3 as I thought.
I always thought it was the other way round in ck2 modding...
eg,
NOT = { 3 }
would mean, less than, and Not equal to 3.
and that,
= 3
meant, equal to 3 OR greater.
Are variables just different?
Or have I been understanding this entire aspect of modding wrong for this whole time? :blush: