There is a paladin somewhere hidden healing himself, he put beacon on him before the duel.
He's too bad of a rogue so he probably needs a pala to heal him so he doesn't lose like every other duel.
But if generally paladin beacon of light isn't supposed to heal while someone is in duel, it is indeed "bug" usage.
Edit: I did some research, Beacon of Light like every other healing spell isn't supposed to heal while someone is engaged in a duel, so this should be considered as bug abuse.
It is both Paladin's and the person with the beacon fault if used this way, because if the person with the beacon chooses not to "abuse" it, they can simply remove it.
Definition of bug abuse:
Someone who uses a exploit to there advantage.
"Bug Abuse is an act of finding a glitch or bug in a game by accident or intentionally, and exploiting it for an unfair advantage in a game."
Beacon of light is not supposed to heal players while they're in duel, like every other healer spell.
Tl;dr this is Bug Abuse.