It's true that 上下 is redundant in this sentence. Sometimes redundancy is used to bring emphasis to certain aspect, as explained in Yi Shen's answer.
There's a term for this linguistic phenomenon: pleonasm. Pleonasm at its face value just means redundancy in linguistic expression. Unnecessary redundancy is of course considered bad style, but when used wisely it certainly can add flavor.
Take for example:
一前一后 and 陆续 both mean "one after another". Adding 一前一后 clearly presents the scene more visually.
Another explanation is related to Chengyu (成语), traditional Chinese idioms that mostly consist of four characters, because four-character group is considered rhythmically beautiful.
There're plenty of Chengyu that are constructed by repetition/redundancy, e.g.:
You could say pleonasm is somewhat prevalent in Chinese, that I barely notice such fact until writing this answer. And in all these examples, the main purpose of repetition is just to form a four-character group.
Padding 上下 to 两层 in order to make it a four-character expression can effectively render it stylistically more beautiful, formal, thus more presentable.
I'd imagine the owner of the house when giving his guest a room tour would say:
But a salesperson when presenting a house to his client would probably say:
The difference is clear, later one is a more formal expression.
Yet another example:
He was so frightened that he went weak in the knees
Both are legit translations, but the later definitely sounds better, while the former feels off. Surely people know man has two legs, so 两 (two) is redundant here. It neither means to disambiguate nor to emphasize, it only serves as a padding word to make it four.