The Rule: Choose words that are exact, idiomatic and fresh. (Harbrace 20, Exactness)
When we write and speak, we must be aware of what words mean. The word "unique" comes from the Latin word "unus," meaning one. E pluribus unum means "out of many, one."
Unique means "being the only one, being without like or equal." A person or thing is either unique or not. You can't be "very unique." You hear it all the time, but this expression doesn't reflect the uniqueness of the word unique.
Another commonly cited word like this is "pregnant." A woman is either pregnant or she is not. She can't be "very pregnant." She can certainly be very far along in her pregnancy, but not "very pregnant."