The first thing to establish is whether they are just duplicated in your library, or are the also duplicated on your disk drive (using Explorer or your favorite file manager). Search around your media. In JRiver you can hover, to see file location. You can also right click the track and on the menu go to 'Locate, On Disk (External)' to jump out of JRiver and into file explorer.
Depending on the result will determine which one you need to clean up.
