A clone in a story is a tool for showing, not telling. They are characters that represent what could, should, or might happen to the protagonist if he or she takes a particular path. Gollum is a clone - he shows what would have happened if Frodo succumbed to the ring.
Not all stories have or need clones. But they are a tool in the toolbox.
A storyteller should know why each character exists! They should not exist merely to decorate the world.