When adding songs that are already added to a playlist, a message will appear that lets you know the playlist contains duplicate songs. To remove all duplicates, follow these steps:
- Sign in here.
- Go to "Your music" and select the playlist you want to edit.
- Click on "Menu", then on "Edit".
- You will see a message saying "duplicate songs found" if your playlist contains duplicates. Click on the red text and select "remove duplicates".