Changes song scheduling to hopefully fix a bug

There was an observed bug, where two songs in a playlist were playing at once after a third song was removed.

src/main/java/net/knarcraft/minstrel/music/Playlist.java

@@ -137,12 +137,8 @@ public class Playlist {
         Song currentSong = this.songs.get(this.currentlyPlaying);
         currentSong.play(trait, trait.getVolume(), trait.getPitch());
         currentlyPlaying++;
-        schedulerId = Bukkit.getScheduler().scheduleSyncRepeatingTask(MinstrelPlugin.getInstance(), () -> {
+        schedulerId = Bukkit.getScheduler().scheduleSyncDelayedTask(MinstrelPlugin.getInstance(), () -> play(trait),
-            if (!currentSong.isPlaying()) {
+                currentSong.getDuration() * 20L);
-                Bukkit.getScheduler().cancelTask(schedulerId);
-                play(trait);
-            }
-        }, currentSong.getDuration() * 20L, 20);
     }
 
     /**

src/main/java/net/knarcraft/minstrel/music/Song.java

@@ -7,6 +7,9 @@ import org.bukkit.Location;
 import org.bukkit.SoundCategory;
 import org.bukkit.entity.Player;
 
+import java.util.HashSet;
+import java.util.Set;
+
 /**
  * A representation of a song playable by a minstrel
  *
@@ -19,6 +22,7 @@ public class Song {
     private final SoundCategory category;
     private final String sound;
     private boolean isPlaying = false;
+    private final Set<Player> playingFor = new HashSet<>();
 
     /**
      * Instantiates a new song
@@ -45,13 +49,15 @@ public class Song {
      * @param pitch  <p>The pitch to play this song at</p>
      */
     public void play(MinstrelTrait trait, Player player, float volume, float pitch) {
+        playingFor.add(player);
         play(player, trait.getLocation(), volume, pitch);
     }
 
     /**
      * Plays this song at the given minstrel's location
      *
-     * <p>Volume of 1 means the max volume, which can be heard up to 1 chunk away. Setting it to 2 means it can be heard 2 chunks away.</p>
+     * <p>Volume of 1 means the max volume, which can be heard up to 1 chunk away. Setting it to 2 means it can be
+     * heard 2 chunks away.</p>
      *
      * @param trait  <p>The minstrel to play this song</p>
      * @param volume <p>The volume to play this song at</p>
@@ -64,16 +70,8 @@ public class Song {
 
         this.isPlaying = true;
         //Mark this song as ended, once 
-        Bukkit.getScheduler().runTaskLater(MinstrelPlugin.getInstance(), () -> this.isPlaying = false, durationSeconds * 20L);
+        Bukkit.getScheduler().runTaskLater(MinstrelPlugin.getInstance(), () -> this.isPlaying = false,
-    }
+                durationSeconds * 20L);
-
-    /**
-     * Gets whether this song is currently playing
-     *
-     * @return <p>True if this song is currently playing</p>
-     */
-    public boolean isPlaying() {
-        return this.isPlaying;
     }
 
     /**
@@ -91,6 +89,11 @@ public class Song {
      * @param player <p>The player to stop this song for</p>
      */
     public void stop(Player player) {
+        //Don't bother stopping if not playing
+        if (!playingFor.contains(player) && !isPlaying) {
+            return;
+        }
+        playingFor.remove(player);
         if (this.category == null) {
             player.stopSound(this.sound);
         } else {