Is there any possibility to add a cover to a playlist?

Question

This means: I dont want to add a cover to every single song in a playlist, but to attach a cover to a playlist as a whole.

Answer 1

I do have many duplicates (one song released in: album of the artist, best of album, compilations, etc.).
To eliminate duplicates: Select Music> File> Display Duplicates. Or use a shareware program such as Song Sergeant, which has the added advantage of allowing you to combine metadfata info (ratings, play counts, etc), before eliminating duplicates.
http://www.lairware.com/songsergeant/
But how to keep the info about album and tracknumber at all and accessible?
The info is kept in metadata tags that is accessible and editable thru the Get Info... window. If you are talking about pretending that the one track you saved comes from all thouse different, multiple sources, your best option would be to add that info to the Comments field. To make this, and other, fields visible in iTunes, simply select them in View> View Options.
My best guess so far is to keep only one file and add that one to several playlists (one for each album the song has been released in).
In a second step I would love to browse through my playlists in cover flow; therefore I would need to add a cover to a playlist. (of course this would need an additional option in coverflow, namely playlists)
That is simply not what Playlists are used for or were desiogned for. Can't be done.
Generally, however, your assumption that all these versions are identical is incorrect. The same song might appear on different albums, but might differ in mix, recording version, remastering version, single mixes, video mixes, etc.