I would generally assume they don't as dots in every other game blizzard has created have not stacked when cast by the same player, only I just fought a mini-boss with haunt and it being my first time using the spell, I figured what they hell why not try? And my dps DID seem to increase dramatically after multiple casts. (I can BARELY run D3 with my current system, so turning on DPS numbers to test this myself is something I'd rather not try.)
Sadly, no - you can't stack the damage over time on Haunt.
This is in the skill's tooltip, although not in the extended tooltip for whatever reason: