This article is from the alt.usage.english FAQ, by Mark Israel firstname.lastname@example.org with numerous contributions by others.
Fowler and other conservatives urge restricting the meaning
of "unique" to "having no like or equal". (OED says "in this sense,
readopted from French at the end of the 18th Century and regarded as
a foreign word down to the middle of the 19th.") Used in this
sense, it is an incomparable: either something is "unique" or it
isn't, and there can be no degrees of uniqueness. Those who use
phrases like "more unique", "most unique", and "very unique"
are using "unique" in the weaker sense of "unusual, distinctive".