The duplicated entries are created by your Mac system. OS X makes quite a lot of use of extended file attributes to store ACLs, metadata, etc. When writing to a file system that doesn't handle OS X file attributes and metadata, it creates a "dot-underscore" file to store that extra information in. On OS X, the "dot-underscore" file is invisible and used in a way that the user is not aware of; on UNIX and Linux, the file is invisible and ignored; on Windows and Android, you see it.
(cf.
https://www.quora.com/Why-does-a-Mac-produce-duplicate-files-when-read-on-a-PC)
Since the content of the files are not relevant to N5ii operation, I suggest you remove them from your DAP or TF cards. These are small files always start with "_" in front of the file name so it should be fairly straight forward to remove them in batch. I have run into similar problem previously and I connect my DAP to a Window PC, conduct a global search for files below 100KB, sort them by filename, and delete all the files started with "-".
N5ii recognize album art by:
- Embedded: you can embedded a JPG file into the header of your music file (if the format supported), this is the best option as you can reorganize your music library anytime and yet the album art will also be available. you can use metedata management software such as MP3Tag to do this.
- Same Filename: In case you want to display a different album art to each song within a folder, you can rename the album art to the EXACT name of the music file, N5ii will change to different album art when they have identify a image file that is named after the music file.
- Predefined: Rename the album art to cover.jpg or folder.jpg and place it into the same folder of the music file, N5ii will look for the file and display it as the album art for all the music files in the same folder.
Please do not use exceedingly large graphic files as album art as they might cause short term CPU brust when loading the graphic file. We recommend jpeg at or below 1000x1000