If I put a playlist on my MP3 player, will the songs go on twice if they're already on there?

My music library is pretty disorganized, and I'd like to just add a few playlists to it like clical that I can listen to when I'm studying, and things like that. Would I have to delete the songs that were originally on there if I put a playlist on it (the only way I know how to make a playlist is to use WMP to make a list and then sync them all over again) or would my MP3 player be smart enough to know that they're the same song? I have a Sansa Fuze. If it would have it on there twice, is there a better way to make a playlist?