Family debate – one says that uniqueness is relative, others say something either is or is not unique. Does uniqueness mean that there is only one of a certain thing/person, so that it would mean more unusual than rare?
I think it is all about usages, that these kind of adjectives are gradable. For example, the meaning of unique is derived from latin unus ‘one’.
So, when its meaning is related to a mathematical concept it cannot be modified. But there is a second sense in unique meaning unusual or special, in this case it isn’t related to the first sense i.e. being only one of its kind.