I've learned that twins in English are always plural, e.g. there are the twins. Then how do we count twins? If there are four people coming, those are two twins? There are two twins? What if there are two people coming, who are twins. There are one twins? Or still There is one twin?
My misunderstanding stems from the fact that the Dutch word "tweeling" (singular) refers to a single pair of twins (two people), and "tweelingen" (plural) would imply at least two sets of twins.