They are unique not as clothing, but as movie memorabilia. If they are off the rack, then as clothing they are 'replaceable' with another pair. As a movie collectible they have tremendous value.

If they are made specific for the film, then they are unique in that light as well. In the original scenario the blouse was sold as one having been worn by princess Di, the fact that it was worn by her,as well as if it had been made special for her would make it unique.