From 80cc890650fe88d381a659ed551d6fdf47978d54 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 e2dd95b..ff7b406 100644
--- a/src/main/java/net/minecraft/server/ChunkRegionLoader.java
+++ b/src/main/java/net/minecraft/server/ChunkRegionLoader.java
@@ -525,7 +525,7 @@ public class ChunkRegionLoader implements IChunkLoader, IAsyncChunkSaver {
     }
 
     public static void a(Entity entity, World world, org.bukkit.event.entity.CreatureSpawnEvent.SpawnReason reason) {
-        if (world.addEntity(entity, reason) && entity.isVehicle()) {
+        if (!entity.valid && world.addEntity(entity, reason) && entity.isVehicle()) { // Paper
             // CraftBukkit end
             Iterator iterator = entity.bx().iterator();
 
diff --git a/src/main/java/net/minecraft/server/World.java b/src/main/java/net/minecraft/server/World.java
index a18a3f6..b7c4e07 100644
--- a/src/main/java/net/minecraft/server/World.java
+++ b/src/main/java/net/minecraft/server/World.java
@@ -1098,6 +1098,7 @@ public abstract class World implements IBlockAccess {
     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
         // CraftBukkit end
         int i = MathHelper.floor(entity.locX / 16.0D);
         int j = MathHelper.floor(entity.locZ / 16.0D);
-- 
2.9.3