How do you guys continuously export tracks/playlists to USB?

Today I tried to export my playlists to USB (on which I had around 400 songs), and after the export, I've got 500 songs, and they took twice as much space. And rekordbox is showing me that I have 1337 songs on my computer. I'm a little bit confused.. I don't really understand what's the best way to export the your library..

Also, I know that in previous versions of rekordbox, you could sync libraries on your computer, and your USB. Now I can't find it.

I'm using 7.2.14.0323 version of rekordbox