Don't add player to world if join event did it already.

On join we unconditionally add the player to the world they logged out in.
If a plugin teleports a player during PlayerJoinEvent in a way that adds
them to a world (cross-world teleport) we end up with one player in two
places. To avoid this we check to see if the player has changed worlds or
is already added to the world we have we skip adding them again.

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

@@ -160,8 +160,12 @@ public abstract class ServerConfigurationManagerAbstract {
         this.cserver.onPlayerJoin(playerJoinEvent.getPlayer());
         // CraftBukkit end
 
-        worldserver.addEntity(entityplayer);
-        this.a(entityplayer, (WorldServer) null);
+        // CraftBukkit start - only add if the player wasn't moved in the event
+        if (entityplayer.world == worldserver && !worldserver.players.contains(entityplayer)) {
+            worldserver.addEntity(entityplayer);
+            this.a(entityplayer, (WorldServer) null);
+        }
+        // CraftBukkit end
 
         // CraftBukkit start - sendAll above replaced with this loop
         Packet201PlayerInfo packet = new Packet201PlayerInfo(entityplayer.listName, true, 1000);