Delete/Remove File From Rekordbox & From SD/USB + Bridge Vs Drag N Drop Duplicates

i'm trying to get into the habit of knowing where to really base my playlists. When I delete something within rekordbox out of a playlist I'm aware that it will remain in your collection to you actually "remove from collection". With that said , If I delete something that I've already synced previously to an SD/USB...when I sync again the file will remain in my SD/USB's collection? Do I have to remember to manually delete each individual before/after syncing or does the sync feature know that the file does not exist in rekordbox collection anymore? Also, Does rekordbox create duplicates if you drag & drop from iTunes vs using the bridge? Seems like sometimes it allows you to add the same exact file to the playlist.