When something is so unnecessary that it could easily be done away with, like a fifth wheel on a car or a fifth person on a double date, call it superfluous.
Superfluous (soo-PER-floo-uhs) means "more than required." Use it when pointing out something that could be removed without detracting from the quality of something: "For a climb over a glacier, the very thickest shoes are absolutely necessary; beyond these, all else seems superfluous to me," wrote the adventurer Charles Stoddard in 1899. The word comes from Latin and literally means "overflowing": super ("over") + fluere ("to flow"). So you can think of a superfluous addition as flowing over the boundaries of what's needed.