That means some people are too lazy or too unskilled to actually double their vocals. So they ask the engineer to do it "artificially", which never sounds as good.
What you do is duplicate the origianl vocal and then do some slight pitch shifteing, some slight delay, maybe some different EQ, so that is sounds like a "different" vocal. It works, but not nearly as well as the real thing.
If you JUST exactly duplicate a vocal, all you get is a vocal track that sounds louder.