From 8a4b7bff6950703ef13ea7ce72e80af4cee67d13 Mon Sep 17 00:00:00 2001
From: Aikar <aikar@aikar.co>
Date: Sun, 10 Jun 2018 20:04:42 -0400
Subject: [PATCH] Properly remove entities on dimension teleport

To teleport an entity between dimensions, the server makes a copy
and puts the copy in the new location, and marks the old one dead.

If this method got called for the same world in the same tick,
the entity would not have been removed from the UUID map, and the
world readd would fail.

This can be triggered even with a plugin if the entity is teleported
twice in the same tick, from world A to B, then back from B to A.

The re-add to A will fail to add the entity to the world. It will
actually be there, but it will not be visible on the client until
the server is restarted to re-try the add to world process again.

This bug was unlikely to be seen by many due to the double teleport
requirement, but plugins (such as my own) use this method to
trigger a "reload" of the entity on the client.

diff --git a/src/main/java/net/minecraft/server/Entity.java b/src/main/java/net/minecraft/server/Entity.java
index 92a15ba947..debab1a715 100644
--- a/src/main/java/net/minecraft/server/Entity.java
+++ b/src/main/java/net/minecraft/server/Entity.java
@@ -2611,7 +2611,7 @@ public abstract class Entity implements INamableTileEntity, ICommandListener, Ke
             }
             // CraftBukkit end */
 
-            this.world.kill(this);
+            this.world.removeEntity(this); // Paper - Fully remove entity, can't have dupes in the UUID map
             this.dead = false;
             this.world.methodProfiler.enter("reposition");
             /* CraftBukkit start - Handled in calculateTarget
diff --git a/src/main/java/net/minecraft/server/WorldServer.java b/src/main/java/net/minecraft/server/WorldServer.java
index e4730352d3..c08ee62e84 100644
--- a/src/main/java/net/minecraft/server/WorldServer.java
+++ b/src/main/java/net/minecraft/server/WorldServer.java
@@ -997,6 +997,7 @@ public class WorldServer extends World implements IAsyncTaskHandler {
     }
 
     protected void c(Entity entity) {
+        if (!this.entitiesByUUID.containsKey(entity.getUniqueID()) && !entity.valid) return; // Paper - Already removed, dont fire twice - this looks like it can happen even without our changes
         super.c(entity);
         this.entitiesById.d(entity.getId());
         this.entitiesByUUID.remove(entity.getUniqueID());
-- 
2.21.0