Duplicate check on playlists.

[ghost]

Checklist

• I checked, but didn't find any duplicates (open OR closed) of this issue in the repo.
• I have read the contribution guidelines given at https://github.com/TeamNewPipe/NewPipe/blob/HEAD/.github/CONTRIBUTING.md.
• This issue contains only one feature request. I will open one issue for every feature I want to request.

Describe the feature you want

If i were to add a video to a playlist when its already in there a duplicate is made. Imo. this is a bit counterintuitive. There could be a warning, when adding the video with confirmations, maybe a popup saying its already in there and not adding a video. Or alternatively moving the video to the end of the playlist.

Is your feature request related to a problem? Please describe it

no

How will you/everyone benefit from this feature?

Easier playlist management for people who use it as a database

[triallax]

I believe is a duplicate of #4186.