For example, you could say "The Mona Lisa is a unique work of art" or "The pyramids of Giza are a unique architectural marvel." You would not say "The Mona Lisa is an in unique work of art" or "The pyramids of Giza are an in unique architectural marvel."
The word "unique" is already strong enough on its own. Adding the word "in" would only weaken its meaning. So, when you want to describe something as special or different, simply use the word "unique."