Add duplicate deletion inside the playlist

[OilSubjectLoss7]

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• I checked if the issue/feature exists in the latest version.
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[eternaltyro]

This would be really nice to have. Basically, what I would like to have is this:

1. If a video is not in a playlist, and I attempt to add it to a playlist (say music), add it.
   • Increment Playlist length numeral
2. If a video is not in a playlist, and I attempt to add it to a playlist, silently ignore and
   • Don't increment playlist length numeral
3. In the information view, if video is added to any playlist, change "Add To" icon to signify, video is already listed. ( + to ✓ or something similar )
4. Even if a video is in some playlist, allow user to add to other play lists. (probably have some indication in the list selection view for whether the video is already in a list)

[Stypox]

@eternaltyro this would be another feature request

@jakerockz97 closing in favour of #2583