Unique means "one of a kind." Therefore, comparatives, superlatives, and words like very, so, or extremely should not be used to modify it. If it is one of a kind, it cannot be compared!
Incorrect: He is a very unique personality.Correct: He is a unique personality.
This same logic applies to other words which reflect some kind of absolute: absolute, overwhelmed, straight, opposite, right, dead, entirely, eternal, fatal, final, identical, infinite, mortal, opposite, perfect, immortal, finite, or irrevocable.
In most cases they cannot be modified by very and similar words, nor can they be used in comparisons.
It is possible to modify the absolute word with something that is not relative. In the above example, you could also say, "He is a truly unique personality." The word truly itself is an absolute term, not a relative one, so it works fine here.
Many times there is a similar modifier which is not absolute. For example, instead of using the word unique in the above example use a similar word that is not absolute.
Incorrect: He has a more unique personality than Marie.Correct: He has a more distinctive personality than Marie.
When an absolute modifier is modified incorrectly, it is sometimes referred to as a qualified absolute.