Reason: Solo - ... - threesome - foursome - group.
The second one probably was couple, but couple soon gained a different definition. Duo was made instead to fill in the gap. Couple means two intimate/cuddly/[insert couple stuff here] characters. Duo means two characters. Please don't aliase, nor delete the tag. I think it's a good way to fill in the gap between solo and threesome, and the use of it should be spread.
Updated by RedOctober