Coming from Japanese here, but I take 产 here to mean property as in 不动产 (lit. “immovable property” i.e. real estate), so 无产 = “no property” (non-owning or more literally non-propertied). The second compound is a pretty common device where you have a simple meaning that could adequately be expressed by a single character, so you just combine two characters with similar meaning so that you can get a two-character “word” (e.g. 決定 which is basically “decide-decide” and means “decide”). I believe one purpose of seemingly redundant compounds like this is disambiguation in spoken language, since either of the characters pronounced by themselves could have many homophones but when combined the meaning becomes clear. There are also cases where it can express a different nuance–in this case, 阶级 specifically means social class, whereas 阶 and 级 by themselves can have much broader meanings of “class” or “rank” (including stair steps, floors of a building, grades, etc.).