Finally someone who ran into the same problem. I think I know where the problem comes from: importing playlists from other devices.
If you have at any point in time imported playlists from any device or tried to "merge" databases, you have to knnow that rekordbox has a maximum of 255 characters when it exports files in the Contents folder on you T5. https://forums.pioneerdj.com/hc/en-us/community/posts/360032463732-Incomplete-file-names-after-export-to-USB-device Meaning, if you have a long artist name and long album name, the path to the track is super long and sometimes file names get crippled. That's where your ghost duplicates are born:
I call this the ghost tracks paradox
I have imported playlists from devices like explained here https://forums.pioneerdj.com/hc/en-us/community/posts/360032463732-Incomplete-file-names-after-export-to-USB-device and implemented them in my master database. I did this because I had lost certain playlists and wanted to put them back in the master database so I could use them again. Only the playlists, NOT the tracks (I do have a single folder /Library on my master database SSD where ALL music files are kept).
The process? As rekordbox had shortened some of the file names in the process of exporting to my USB for the /Contents folder, importing the playlists back from the USB inevitably creates duplicates of some tracks because they have a different file-name now compared to what's stored in my master database's folder. Because of the 255 character maximum, there are now two instances of a track, one file path is in my master-folder just Library/Darude - Sandstorm.mp3 and the other on the USB LongAlbumName/Darude - San.mp3
The problem? Once I disconnect the USB, it's files obviously end up as missing. But when I try to relocate them because, hey, of course if have them in my master library and all I wanted was the playlists sorted data, it creates ghost tracks: I am looking for a file for Darude - San.mp3 and click on Darude - Sandstorm.mp3 in my library. This actually works, the files starts playing in rekordbox,
BUT: There are now two instances of Darude - Sandstorm that are linked to the SAME mp3 and play just fine. But I don't want duplicates while browsing on the CDJ. I could go and delete all those duplicates by hand, but before I'll have to drag them manually into playlists. Defeats the purpuse, but okay, no other option. But here it get's messy:
After having deleted the duplicates, they STILL keep showing up on CDJs and XDJs (+RX2) - and one of them comes up as "missing". It seems rekordbox is unable to "forget" that there have been two instances of the same track in the collection at one point. And there is no way of getting rid of them. No matter what I do, on rekordbox I only have one instance of Darude - Sandstorm, but two on the CDJ's browser.
Matter of fact, this irreversibly messep up my whole library and I am currently sorting this out - by hand. To make things worse, have the feeling that rekordbox doesn't export questionmarks in the file names, or is somehow confused by how to read them. Problem seems connected, but I have to go now, sorting my library.