Anne:
You will need to use the Finder/Explorer to search inside the folders that contain the libraries to find and eliminate the duplicates. If the libraries are Containers (.lcf files), then you will need to Extract the libraries first to work inside of them.
You should add a Signature to your Profile (click the Profile button near the top of this page) with your ArchiCAD version and operating system (see mine for an example) for more accurate help in this forum.
David