If it's from lag I usually just keep on playing--the other person's probably getting it too, and I'll have to put up with it for the rest of the game anyway. A lot of players will stop the duel in the case of fake health from fake carrots and RFs, however--it's pretty easy to tell when someone has fake hearts in those cases (just shoot the person complaining). Since that works both ways, I'm amenable enough to killing the other player and letting them capture the flag again. 3HK's are annoying, after all.

(It's just nicer to say you have fake health before you die, not after.)