3.5.8213 P
Unless an additional setting has been added that I'm not aware of or something else has changed, it seems a warning is no longer presented when a duplicate song is added to a playlist. The duplicate does get added and "1 file added" shows up in the status at the bottom, but no warning.
I've tried it with songs that have the same title/artist but different file names, and even with the actual same file, and neither presents the warning.
I do have the option to warn checked in confirmations.