A singleton has an established meaning in programming and mathematics, but I'm curious about the more general use of the term (as defined by Merriam-Webster):
2a : an individual member or thing distinct from others grouped with it
Suppose I have a group member that is not distinct from the other group members. What is the opposite of a “singleton member”? Google results are surprisingly unhelpful. The closest I can get is this SO answer, specific to the programming context.