Thing with duping is,
If its overdone its bad.
example: if u need 3 and u dont have the money to buy them and u dupe 3 its not a big of a deal(the selller wouldnt even notice)
on the other hand, if u have the money but dupe instead then the "seller of the stones" would be negatively impacted.
And as for duping stones to gain gold: thats a bad thing, couse prices of not dupable items will rise, couse no one needs the duped items since they can make them, as for wc(for example) the prices will inflate like mad since there is almost every1 that can buy it, so prices go up.
so to be short, duping is bad for 2 thinks: webzen(couse they sell less chests etc couse of the fact u can get things for free) and for the ppl who dont know how to dupe, or inviested all there gold in the duped item.