If you mean that you want all the storage components of a FCP library: Media, Motion Content, Cache, Backups all duplicated to a new location then in the original library change the storage locations to a folder in the original drive. Then duplicate the folder in the finder and put the duplicate where you want the copy. CCC is a solid app that lets you make backups and will modify updates without having to duplicate the drive or folder from scratch. It will also come in handy when doing a backup of any drive or folder that you have.
If what you meant was just the project (timeline) why not just export it if all the editing is done already?
Imacs (i7), Canon C300, Canon 5D Mark IV, Panasonic ENG HPX250P, , FCP X, teach video production in L.A., Cool Light Productions, Producing series of multimedia Portraits of creative women in the production arts.