I see, thanks, tracked as: https://www.ventismedia.com/mantis/view.php?id=15263

Tools > Options > Library > Tags & Playlists > Playlists > Warn When Duplicate Files are Added

There is no such option. Do you mean "Analyze files for duplicates"? This option means, that files are analyzed and you can see duplicates in Files to Edit -> Duplicate Content view after scan, where you can decide what to do with duplicates.

The issue has been present in all tested builds including 2138. Even though the option is checked in settings, adding duplicate tracks is still allowed. Noted issue in 2134 and have verified still present.

Tested with option unchecked and reactivated. Issue persists with a clean install and with an optimized library.