It's not quite a synonym for "different", though. If you say "that's a very different project", there has to be some context for what it's different from; it can't stand alone, except in some colloquialisms. On the other hand, if you say "that's a highly unique project", it's understood that it's very different from 'most of the (actual) other projects'... and aside from being verbose, that formulation is only an approximation to the meaning of "unique". For one thing, "unique" suggests a holistic comparison among the whole class, while something like "different from most" suggests a series of two-way comparisons (is it different from A? is it different from B?), each of which can only be yes or no, and "most just counts how many of the results are yeses. This is not necessarily what is intended, since the meaning can be corrupted into 'different from [the general characteristics of a group containing most of the others]', but I'd call that more abusive than just grading "unique". And even that changes the meaning of the grade: if something is 'extremely different from most of the others', then it can still be identical to a decent chunk of the others; the adverb only applies within the aforementioned comparison(s) (even if to a group). If something is extremely unique, then it's probably different from almost all of the others, but chances are it's also very different from most of them; the adverb applies to the holistic comparison.