If we use the same monster to upgrade the hero, it should increase the skills as well. Too much to get double the hero just to skill and upgrade the rank.

Example. Gorgorab is default 4 star unit. It make him 5 star, we need 4pcs 4 star units right? But if I use 4 pcs Gorgorab to make him 5 star, it should level his skill as well.