Made teleporting between worlds 'smoother'. Thanks Raphfrk!

src/main/java/net/minecraft/server/PlayerInstance.java

@@ -37,8 +37,11 @@ class PlayerInstance {
         if (this.b.contains(entityplayer)) {
             throw new IllegalStateException("Failed to add player. " + entityplayer + " already is in chunk " + this.chunkX + ", " + this.chunkZ);
         } else {
-            entityplayer.g.add(this.e);
-            entityplayer.netServerHandler.sendPacket(new Packet50PreChunk(this.e.x, this.e.z, true));
+            // CraftBukkit start
+            if (entityplayer.g.add(this.e)) {
+                entityplayer.netServerHandler.sendPacket(new Packet50PreChunk(this.e.x, this.e.z, true));
+            }
+            // CraftBukkit end
             this.b.add(entityplayer);
             entityplayer.f.add(this.e);
         }
@@ -63,7 +66,7 @@ class PlayerInstance {
             }
 
             entityplayer.f.remove(this.e);
-            if (entityplayer.g.contains(this.e)) {
+            if (entityplayer.g.remove(this.e)) { // CraftBukkit - contains -> remove
                 entityplayer.netServerHandler.sendPacket(new Packet50PreChunk(this.chunkX, this.chunkZ, false));
             }
         }

src/main/java/org/bukkit/craftbukkit/entity/CraftPlayer.java

@@ -12,6 +12,7 @@ import net.minecraft.server.Packet54PlayNoteBlock;
 import net.minecraft.server.Packet6SpawnPosition;
 import net.minecraft.server.ServerConfigurationManager;
 import net.minecraft.server.WorldServer;
+import net.minecraft.server.ChunkCoordIntPair;
 import org.bukkit.Achievement;
 import org.bukkit.Location;
 import org.bukkit.Material;
@@ -194,6 +195,16 @@ public class CraftPlayer extends CraftHumanEntity implements Player {
             if (teleportSuccess) {
                 manager.server.tracker.trackPlayer(entity);
                 manager.server.tracker.untrackEntity(entity);
+                int cx = (int) location.getBlockX() >> 4;
+                int cz = (int) location.getBlockZ() >> 4;
+                for (int x = -10 ; x <= 10 ; x++ ) {
+                    for (int z = -10 ; z <= 10 ; z++ ) {
+                        ChunkCoordIntPair chunkPosition = new ChunkCoordIntPair(cx + x, cz + z);
+                        if (entity.g.remove(chunkPosition)) {
+                            newEntity.g.add(chunkPosition);
+                        }
+                    }
+                }
                 oldWorld.manager.removePlayer(entity);
                 manager.players.remove(entity);
                 oldWorld.removeEntity(entity);