A function that removes missing files from the playlist has been in Nemp for a while. With version 4.15 a search for duplicates is implemented as well.

The playlist is scanned for potentially duplicate tracks. Nemp distinguishes between a possible duplicate and a definite duplicate:

An entry is a possible duplicate if the file name matches (but in different directories), or if artist and title match.

An entry is a definite duplicate if both entries point to the same file.

In the screenshot, the first song is highlighted in the playlist for which a duplicate was found at position 10 and 19. For the 10th song in the playlist the file name and artist and title match, the 19th track is a clear duplicate. (In fact, all possible duplicates here are the instrumental versions of the Earbook edition of the album.)

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You can then step through all the possible duplicates and delete the original entry in each case, or delete the duplicates from the playlist.