Display a warning message when you try to send a song/album/artist that you've already sent to someone.
I share favorite tracks and new finds with a group of people quite often and I can never remember which songs I've sent to them already and which I haven't. I wouldn't want to spam their Spotify inbox with the same song that I've already sent them before, so a warning message would come in handy to let me know not to send it again!
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