Wikipedia’s article on Osama bin Laden’s compound in Abbottabad, Pakistan quotes “US officials” describing it as “extraordinarily unique”.
Some style guides advise or prescribe against any modification of unique. Either something is the only one of its kind, or it’s not. It can’t be (for example) very unique. While modifying unique is probably best avoided in formal contexts, there can be no doubt that many people say or write it informally and normally. Google Ngram Viewer shows not (by far the most common), very, as, most, so, quite, rather, somewhat, almost and probably unique. Some of these are (probably) more acceptable, and others less so.
Extraordinarily unique isn’t on Ngrams’ top 10 results (its usage is about one-tenth that of probably unique), but a general Google search shows about 391,000 results, starting with blind auditions on The Voice, the Atlanta Motor Speedway and the Villa Bismarck on Capri.
It might just be possible to describe something as extraordinarily unique if it’s extraordinary as well as unique – a whole level more unique than anything else. Australia has many unique animals, but the platypus is extraordinary. Anyone familiar with jerboas will accept the kangaroo, but when the first samples of dead platypuses (?platypi, ??platypodes) arrived in England, the experts there thought someone here was playing a practical joke on them. But “except for its size and exaggerated security measures,” Bin Laden’s compound “itself did not stand out architecturally from others in the neighbourhood.”