How many time a recommender system can recommand the same item to an user?
I don't think it matters if you recommend the same song in multiple playlists, just that your recommendations were accurate.

For example, imagine a user has two playlists a rock playlist and a classical playlist. If you recommended Crazy Train - Ozzy Osbourne in both playlists, you could theoretically have a "successful recommendation" in the rock playlist, and an bad recommendation in the country playlist.

The number of times a song appears is a design decision/variable - there is no right answer. You could potentially optimize on in the future using A/B testing (if I limit a song to being recommended x times, do those users then value my recommendations more than the general population?).