The issue is because there are different valid definitions of “unique”, with some people desperately trying to fight against the evolution of language to claim otherwise 
Can something be very unique or somewhat unique?: Usage Guide
Many commentators have objected to the comparison or modification (as by somewhat or very ) of unique , often asserting that a thing is either unique or it is not. Objections are based chiefly on the assumption that unique has but a single absolute sense, an assumption contradicted by information readily available in a dictionary. Unique dates back to the 17th century but was little used until the end of the 18th when, according to the Oxford English Dictionary, it was reacquired from French. H. J. Todd entered it as a foreign word in his edition (1818) of Johnson’s Dictionary, characterizing it as “affected and useless.” Around the middle of the 19th century it ceased to be considered foreign and came into considerable popular use. With popular use came a broadening of application beyond the original two meanings (here numbered senses 1 and 2a). In modern use both comparison and modification are widespread and standard but are confined to the extended senses 2b and 3. When sense 1 or sense 2a is intended, unique is used without qualifying modifiers.
There is a handy list of words prescriptivists are trying to freeze in time without success:
