When adding an album to a playlist, the feature excluding songs already in the playlist is gone. It used to pop up saying "Do you to exclude songs already in playlist" or something to that effect, but it doesn't anymore.
There's actually no notification confirming the songs have been added to playlist, which I'm sure used to pop-up as well.
The duplicate warning should pop up when adding whole albums to a playlist, if songs from it are already included. If that's not the case, it's possible that some of the app files on your device are damaged. An easy way to remedy this is to perform a clean reinstall - this ensures the integrity of all files, as well as updates the app to the latest version available.
If doesn't fix thing, could you test if the warning appears on another device or when using the web player?
Hope this helps. Keep us posted on how it goes.
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