Thanks for reporting, it was recent regression, fixed as https://www.ventismedia.com/mantis/view.php?id=20548

Yes, the option is still enabled. The problem isn't not getting a confirmation - I do still get a confirmation popup. It's just that previously I'm pretty sure it listed the duplicated tracks within the popup box like this so you knew which tracks you were about to duplicate in the playlist:

[url=https://ibb.co/LYWYvRG][img]https://i.ibb.co/gmpm69n/Untitled.jpg[/img][/url]

(I've typed in the circled text as an example of where it used to list the track names!)

Is the option to do so still enabled under Tools > Options > Confirmations or did you disable it?

I'm pretty sure that in previous builds if you try to add files to a playlist that are already on that playlist it would display a list of those existing files in the popup. But in the most recent build (2829) it no longer does:

[url=https://ibb.co/rdSpz1r][img]https://i.ibb.co/g4pWfBK/Untitled.jpg[/img][/url]

Shouldn't it show existing playlist entries after the text "Duplicate file(s) detected:"?