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.

Definitions of superfluous

adj more than is needed, desired, or required

“delete superfluous (or unnecessary) words”
excess, extra, redundant, spare, supererogatory, supernumerary, surplus
unnecessary, unneeded
not necessary

adj serving no useful purpose; having no excuse for being

otiose, pointless, purposeless, senseless, wasted
lacking in usefulness or value