Since installing AS 1.7 the duplicate tracks problem has returned to my local library. Not all albums just a selection that always seem to be affected. This fix posted (AS 1.6.4.6_m) seemed to work but after upgrading I’ve had 1 album with all tracks duplicated and today 2 further albums with only certain tracks duplicated. Were the fixes released with the test version incorporated in 1.7?
After the problem got worse with each sync/restart I followed the advice given by @RunHomeSlow under Library Rebuild
I also changed my synced folder to a single location (4 Previous) and turned off Sync on Restart.
For the last few days the library has behaved itself. 
Yes, 4 Bay Synology with a dedicated bay and music drive. As of now the library is still good.
My TIDAL playlists remain with duplicate entries as noted above. However, when I pull the TIDAL playlist to a local playlist, there are no duplicates!
However, any changes to either the TIDAL playlist or this local Playlist (which is a copy of TIDAL’s) don’t sync, so I need to manually re-sync the playlists, which is very inconvenient.