New playlists

I’m new to mp3 players. I have playlist A on the Sandisk player originally made with Windows Media Player. I then added songs to playlist A with MediaPlayer, then re-synced playlist A to the Sandisk player. Would the process realize the revised playlist A contains files already on the Sandisk player and just add the newly added songs? Or - should I delete the playlist and/or music files from the Sandisk player first? I realize a playlist doesn’t contain the files, but a reference for the player where to find the files in a designated order. 

I got this player with acrued points from my credit card company. So far I’m very impressed. So much easier to use than the Zoom mp3 stereo recorder I was using as an mp3 player. 

if you sync using windows media player it should see the songs that are already on it and you will see already on device as the status as it skips the song.