When King Piccolo was dying he basically put his essence into Piccolo Jr., gave him all of his memories, feeling, etc. Which is why Piccolo hated Goku so much at first, but other species reproduce asexually and it does not mean they clone themselves, they just create another creature in a sense. With Namekians its different, because they are more like plants then anything, Piccolo really only lives off water, and regrow limbs, etc. They are like a cross between lizards and plants.