Lets say you are an undead slayer.
Lets say you are fighting an undead that is a you slayer.

Suppose both of us have x hunter (Slay unblockables).

I know that you cannot use skills to negate an unblockable tagline; yet, the cloudy question is:

If both are slayers of each other and an unblockable slay is thrown, could the other use their riposte being that their slays are also unblockable?

If no, then I guess its "whoever throws the first slay" wins any matchups.

As of now, it would go to whomever hit first.

As far as I can tell, it has never been the case that the free riposte is allowed to be used. It makes sense that they have trained to be awesome at killing their chosen Foe, but it stretches reason that they trained to be awesome at defending from their chosen Foe that has trained to be awesome at killing them. Basically, they can each kill each other very well, but are subject to the same attacks that kill them really well.

I think it should be a defensive toss at that point. Who has the most HP. It keeps from adding specific erratas to rules.

As of now, no errata is needed - just do as the skill says.

And, like Brian says, most fighters that have these packages also have Chart 3 with resilience and can eat a slay.