From 8adb94be4ba737d0211814d78363b2b3927a9b0a 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/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index d0ab36190..4ba5b00ae 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -937,6 +937,7 @@ public class WorldServer extends World {
     // CraftBukkit start
     private boolean addEntity0(Entity entity, CreatureSpawnEvent.SpawnReason spawnReason) {
         org.spigotmc.AsyncCatcher.catchOp( "entity add"); // Spigot
+        if (entity.valid) { MinecraftServer.LOGGER.error("Attempted Double World add on " + entity, new Throwable()); return true; } // Paper
         if (entity.dead) {
             // WorldServer.LOGGER.warn("Tried to add entity {} but it was marked as removed already", EntityTypes.getName(entity.getEntityType())); // CraftBukkit
             return false;
-- 
2.22.0