The same BUG in version 4.1.28.1901.

SOLUTION: Additional option - Save this answer and stop asking

1. Make a new playlist.
2. Drag some songs onto it.
3. Drag one of those songs onto it again.
RESULT: You are asked if you want to add the duplicate or not, and if MediaMonky should bother asking in the future.
4. Indicate "Don't Ask Again" and "No, don't add the duplicate".
5. Drag another duplicate on.
RESULT: Notice that it didn't ask (good), but it added the duplicate (bad).
EXPECTED: If I said don't add duplicates and don't ask again, it should automatically not add duplicated in the future.

Thanks.
(Using mm 4.1.27.1987 on fully updated Win 10)