[Feature Request] Dont add duplicate songs

### Is your feature request related to a problem? Please describe.

When you add multiple playlists with some songs overlapping the songs get added twice (or even more often)

### What is your ideal solution to the problem?

Dont add a song if it is already in the queue

### How would this feature be used?

1. Add one playlist
2. Add second playlist
3. Only songs who are not already in the queue are getting added

### Additional Info

_No response_

### Checklist

- [X] I have checked [the documentation](https://jmusicbot.com/) to make sure this feature doesn't already exist
- [X] I have searched for [similar feature requests](https://github.com/jagrosh/MusicBot/labels/enhancement)
- [X] I have confirmed that my requests is **not** on the [list of things that will not be added](https://jmusicbot.com/things-that-wont-be-added)
- [X] I am running the latest version of the bot: 

Start by tracing how playlist additions enter the music queue and where queued songs are compared. Reproduce the two-playlist sequence described in the issue, then verify that songs already in the queue are skipped while new songs are added.

Written by the indexing model from the issue text.