So, I'm playing a Heritech follower of Slaanesh.

You may think that is odd, but, hey, you haven't truly experienced music until you've heard the symphony of gamma radiation released by a tactical nuclear weapon - provided by your implanted auspex, of course.

But, I've managed to snag best quality cybernetic senses, meaning my character gets Heightened Sense (All). Then I read the bonuses for a mark of Slaanesh...which gives you Heightened Senses (All).

Do...those two things stack? Or is that bonus lost?

Well, as Heigthened Senses (Any) can't be acquired twice for the same sense, I'm assuming your bonus is lost.

Daaaaaang iiiiiiiiit!

RAW no.

I'd probably consider allowing a partial broadening of those senses, though, if only in an rp sense. Instead of simply 'hearing' sounds, you might be able to "see" sound, or to 'smell' light.

I might also allow using them as the basis for the development of Unnatural Senses.

The flip side is, you're going to get bored with various sensations a whole lot faster.

Something similar to Synesthesia