If you are looking for a way to add songs to Playlist without adding them to Library, it is likely that the Library in your Apple Music Account is riddled with duplicate songs.
This duplication of songs in Apple Music Library happens due to the default tendency of the Music App to automatically create a copy of songs that are added to the Playlist.
In addition to creating a clutter of duplicate songs, this default setting in Music App leads to the issue of songs that you have already played in the Playlist repeating and playing once again when you play them on shuffle in Apple Music.
Add Songs to Playlist Without Adding to Library in Apple Music
The solution to the issue of duplicate songs in Music Library is to find a way to prevent the Music App from making a copy of the songs that are added to the Playlist.
Luckily, this can be easily achieved by deselecting the default “Copy files to Music Media Folder when adding to Library” option on a MacBook and disabling “Add Playlist Songs” to Library option on iPhone.
1. Add Songs to Playlist Without Adding to Library On Mac
The easiest way to prevent added Playlist Songs being automatically copied to Music Library is by going to Music App Preferences on a MacBook.
1. Open the Music App on your Mac > click on the Music tab in the top-menu bar and select Preferences in the drop-down.

2. On the next screen, switch to Files tab and uncheck Copy files to Music Media Folder when adding to Library option

3. Click on OK to save this setting for Apple Music on your MacBook.
After this change, you should be able Add Songs to Playlist without worrying about the added songs also being copied to your Music Library.
2. Add Songs to Playlist Without Adding to Library on iPhone
If you have subscription to Apple Music, you can follow the steps below to prevent songs added to Playlist being automatically copied to your Music Library.
1. Go to Settings > Apps > scroll down and select the Music App.
2. On the next screen, scroll down to Library section disable Add Playlist Songs option.

After this change, you should no longer find songs added to the Playlist being copied to your Music Library in Apple Music.