I think the best thing to do is to treat fake health kills like normal kills. People lie, and besides the damage is done whether the person knows it is there or not. Sure, if you are at five lives, you're in trouble, but this is a price that must be paid.

Although this can be caused by RF's, lag is also a factor. If this was caused exclusively by lag, you may want to consider whether the duel is too lagged to be valid anyway.