From 0000000000000000000000000000000000000000 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 f99bb7f52254d7f052ecfce2b94da329a9d05eea..b68338310106858195099bb45b8bf4da723b8439 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -919,6 +919,7 @@ public class WorldServer extends World implements GeneratorAccessSeed {
     // 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;