Then I got a lot of duplicate files. For example in subfolders relating to my mp3 player I even discovered my whole NextCloud ‘drive’ is there again: /run/user/1000/doc/by-app/org.strawberrymusicplayer.strawberry/51b78f5c/N
In Linux a file can show up several times in the filesystem without being duplicated. Symbolic links and hard links will cause this to happen, and they’re a normal part of organizing the filesystem. Just because you see a file in several places, that doesn’t mean disk space is wasted with duplicates. There may be only one physical copy of the file, appearing in multiple places. With hard links you need to be especially careful about deleting, since you’ll think you’re deleting one of several duplicates but you’ll in fact delete the only copy of the file.