Don't leave the NearbyPlayers tracker in an entirely busted state on double-add detection

patches/server/MC-Utils.patch

@@ -5520,7 +5520,7 @@ index 0000000000000000000000000000000000000000..00000000000000000000000000000000
 +
 +    public void addPlayer(final ServerPlayer player) {
 +        final TrackedPlayer[] newTrackers = new TrackedPlayer[TOTAL_MAP_TYPES];
-+        if (this.players.put(player, newTrackers) != null) {
++        if (this.players.putIfAbsent(player, newTrackers) != null) {
 +            throw new IllegalStateException("Already have player " + player);
 +        }
 +