They can be commented and voted on, but they can't actually be answered. What you might be thinking of is the "possible duplicate" stage, where a person has suggested that it might be a duplicate, but the community hasn't yet confirmed it. It's just like casting a close vote (it is, in fact), except it's made public so that the user--assuming it is actually a duplicate--is given their answer more quickly.
The reason duplicates aren't (generally) deleted is so that they can act as sign-posts, directing further traffic to the original question. They can be commented on just like any other closed post, because people might say "hey! this isn't a duplicate, [here's why]." They can also be voted on, because just because something is a duplicate doesn't mean it's exactly the same in presentation or research. There's no good reason to lock a question just because it's been marked as a duplicate.