A superfluity is an excess or abundance. When you encounter superfluity, there's too much of something.
When something is superfluous, it's unnecessary or redundant: there's already enough of it. Likewise, superfluity is too much of something. If your friend is a shopoholic and spends all of her time at the mall, you probably encounter superfluity in her closet — since she has more clothes than she will ever wear. The Latin root word is superfluus, which is used figuratively to mean "unnecessary," but is literally "overflowing."