From 6cfcac4ce43f629bdd36407b5450ec87b0b9d986 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Tue, 21 Jun 2016 22:54:34 -0400
Subject: [PATCH] Fix Double World Add issues

Vanilla will double add Spider Jockeys to the world, so ignore already added.

Also add debug if something else tries to, and abort before world gets bad state

diff --git a/src/main/java/net/minecraft/server/ChunkRegionLoader.java b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
index f007af2e1..2f1488ee5 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -1050,7 +1050,7 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
     }
 
     public static void a(Entity entity, GeneratorAccess generatoraccess, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason reason) {
-        if (generatoraccess.addEntity(entity, reason) && entity.isVehicle()) {
+        if (!entity.valid && generatoraccess.addEntity(entity, reason) && entity.isVehicle()) { // Paper
             // CraftBukkit end
             Iterator iterator = entity.bP().iterator();
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index b6efd8506..e74b53c48 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -966,6 +966,7 @@ public abstract class World implements GeneratorAccess, IIBlockAccess, AutoClose
     public boolean addEntity(Entity entity, SpawnReason spawnReason) { // Changed signature, added SpawnReason
         org.spigotmc.AsyncCatcher.catchOp( "entity add"); // Spigot
         if (entity == null) return false;
+        if (entity.valid) { MinecraftServer.LOGGER.error("Attempted Double World add on " + entity, new Throwable()); return true; } // Paper
 
         org.bukkit.event.Cancellable event = null;
         if (entity instanceof EntityLiving && !(entity instanceof EntityPlayer)) {
-- 
2.18.0