Okay, I noticed this yesterday and my friend also commented on it. Me and my friend were in a party. My friend dueled another person, I also dueled someone.
I put a PoM(Prayer of Mending) on myself and got attacked which caused it to jump to my dueling friend.
Since you're not usually able to heal people who're dueling, I guess this is considered an exploit.
I haven't have time to test this again though so a confirmation would be good.