Duplicates when importing/opening playlist

I have my library set up to watch for file changes and when I open a playlist I found duplicates in my library from the playlist I opened/imported. I've exported my playlist directly from Nightingale, and the path are absolute.
I've tried two different approaches. I first let my library get populated with all my music and then import the playlist which obiously create duplicates.
The second case, I deleted all my music from the library, turn off the watch-for-changes option, import the playlist first and then scan for my files. In this way, I thought that if a file is already in my library after importing the playlist, it wouldn't be added later when the watch-for-changes option is on. I still found duplicates of my files. I don't know why Nightingale doesn't check if the files are already in the library and use those already instead of importing them, for either case (importing playlist first or last).

I installed The Exorcist add-on so I can track the duplicates, but I don't know which one to delete, since both of them are the same song pointing to the same file. By deleting one of them, I could be deleting the file from my playlists, so it's impossible to do something about it.

I've been trying to open the sqlite files to see where the playlists are stored, maybe I can add them manually there without importing them and creating duplicates. This is just a workaround and it's not the way to go at all...

I don't find any error in the error console, or anywhere else so I don't know which log to attach.