Brokeleg Benny, the gladiator, has a twin.
It seems there are two versions of him in rotation.
And sometimes they both spawn.
Maybe he's just quicker at moving from one arena to the other than you are.
(Seriously, while I understand it strains creduility a bit, why is this such a big deal? I think it's cool that some NPCs visit both arenas sometimes and while an unfortunate side effect of that is that sometimes they will spawn at both is it really such a big problem?)
No I think there are literally two versions of him in rotation.
There were 2 of him in the same place and they are counted as different gladiators as you can fight both for 2x the XP reward.
I also think they are slightly different mechanically
Both versions of him together at the same time, not a big issue but stil bloth are treated as different NPC for duelling purpouses:
https://imgur.com/DyWa0mr (https://imgur.com/DyWa0mr)