[Fela]

The avoid duplicate songs feature does use the full file path, not the song name. However, the default storage setting of "Use Subdirectory per Song" uses the song title as the folder name, so naturally this affects the duplicate file detection.  My advice would be to back up your library, then restore that backup file (which will move all of the files into the appropriately named directories), so that the avoid duplicate songs feature works from that point onward.

Restoring the Backup won't rename the songs to the much nicer interspaced names though, will it?
(Still considering that on my primary tablet for the annotations...)