From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sat, 21 Jul 2018 08:25:40 -0400
Subject: [PATCH] Add Debug Entities option to debug dupe uuid issues


diff --git a/src/main/java/net/minecraft/server/level/ServerLevel.java b/src/main/java/net/minecraft/server/level/ServerLevel.java
index 425c22fe94e83d880b331cbfb16dc67f22def1c7..a12d5de8834bbc3176be3d2c8353b2d4372dbc1d 100644
--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
+++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
@@ -1196,6 +1196,12 @@ public class ServerLevel extends Level implements ServerEntityGetter, WorldGenLe
     // CraftBukkit start
     private boolean addEntity(Entity entity, CreatureSpawnEvent.SpawnReason spawnReason) {
         org.spigotmc.AsyncCatcher.catchOp("entity add"); // Spigot
+        // Paper start - extra debug info
+        if (entity.valid) {
+            MinecraftServer.LOGGER.error("Attempted Double World add on {}", entity, new Throwable());
+            return true;
+        }
+        // Paper end - extra debug info
         if (entity.isRemoved()) {
             // WorldServer.LOGGER.warn("Tried to add entity {} but it was marked as removed already", EntityTypes.getKey(entity.getType())); // CraftBukkit
             return false;
diff --git a/src/main/java/net/minecraft/world/level/entity/EntityLookup.java b/src/main/java/net/minecraft/world/level/entity/EntityLookup.java
index 38df704dca30ef08f4d0831dc1cc48c6d6f71a4d..ed6aea7a38ef6e80c300ff9b012dcdbc390ad2c7 100644
--- a/src/main/java/net/minecraft/world/level/entity/EntityLookup.java
+++ b/src/main/java/net/minecraft/world/level/entity/EntityLookup.java
@@ -33,6 +33,14 @@ public class EntityLookup<T extends EntityAccess> {
         UUID uUID = entity.getUUID();
         if (this.byUuid.containsKey(uUID)) {
             LOGGER.warn("Duplicate entity UUID {}: {}", uUID, entity);
+            // Paper start - extra debug info
+            if (entity instanceof net.minecraft.world.entity.Entity) {
+                final T old = this.byUuid.get(entity.getUUID());
+                if (old instanceof net.minecraft.world.entity.Entity oldCast && oldCast.getId() != entity.getId() && oldCast.valid) {
+                    LOGGER.error("Overwrote an existing entity {} with {}", oldCast, entity);
+                }
+            }
+            // Paper end - extra debug info
         } else {
             this.byUuid.put(uUID, entity);
             this.byId.put(entity.getId(), entity);