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 b9107796833f036977dab4e388ce822307985e20..9599f1141c90e803ca0a9cd9b15ed1205beca8eb 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -927,6 +927,7 @@ 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;
