Under "wounds and removing casualties" it states that multiple causing wound weapons only affect models that have more than one wound. it also clearly states that any extra wounds on a model (not a character in a challenge where the overkill rule plays into affect) with only 1 wound, will be discarded and will not count towards your combat RES.
for example :
squad of Empire go up against a chaos lord with hellfire sword (d6 wounds after initial wound)
the Chaos lord kills 1 spearman, he does not get to roll again for d6 wounds on that model with only 1 wound.
If the pikemens sergant were to challenge the Lord, once the lord inflicted one wound he would then roll a d6 for mutiple wounds since the sergent has 2 wounds, ie: getting a lot of overkill wounds.
This same effect comes into play vs. anything with multiple wounds, however its a wasted amount of points on a weapon you can only use in this fashion, unless you have some wicked plans :twisted: