This glitch happened to me when transporting my horse across nether portals. After transporting my horse from the nether to the overworld there was about a 10 second delay where my horse seemed to be no where and then it spawned into the overworld. After moving my horse to an area where it could not reach the nether portal I went back into the nether and my horse was magically still in the nether. I put the horse back in its pen in the overworld without problem the second time. 10 minutes later I am using a different nether portal and my horse is found magically next to this separate nether portal that is in the nether. I transport it to this nether portal's overworld and it suffocates and dies in the blocks surrounding this overworld portal. I return to where I had left my horse about 10 minutes ago, in its pen, and it is now there. I was holding the saddle and leather that I got from the horse's death while looking at the same horse in the pen. I walked around the village where I kept my horse and the villagers that I had nametagged were duplicated and my singular black cat that I left leashed to a fence in my house was now three and the lead had also duplicated. Sorry this is so long, I felt it would be better if you had enough information to duplicate this scenario completely.
10 minutes after writing what is above: Glad I did more testing, the first part of this was all done in survival with cheats enabled via "Open To LAN", I closed the game completely and reopened and now all duplicated mobs are gone, all except for a villager that I nametagged, got duplicated and then became a zombie villager. The horse is still in its pen, I guess if there are duplicated mobs and you reopen the game, the unoriginal will despawn, also I'm guessing that the zombie villager didn't despawn because the game didn't see him as a duplicate anymore.
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Johnibur
Might be related to: https://bugs.mojang.com/browse/MC-138911
Johnibur
Can confirm with 18w46a + more info on how to reproduce it with detail:
Make any mob going to the Nether by using a Nether portal, go to the Nether, you'll see one single mob on the other side, go back from the Nether, then go the the Nether again. This time, there will be 2 versions of the same mob, which are perfectly usable (horses can be mounted, villagers can be traded with).
I tried with a chicken, a horse and a villager and it duplicates the mob using the same method every time. In the logs, I have the following warning, one for every type of duplicated mob every time I load the concerned chunk:
[Server thread/WARN]: Keeping entity minecraft:villager that already exists with UUID […]
Can also be extended to items on the ground, the item will duplicate the same way, but the duped one cannot be picked up.
Now if I relog, the duplicated mobs will not be rendered anymore, but I still get the same warnings about entities with same UUID on the console.
Johnibur
This is still an issue with 18w50a.
Here are the logs of a newly created world after creating a portal to the Nether and pushing a villager through it, then coming to the Nether, coming back, coming to the Nether again (items on the ground duped), and finally coming back to the overworld (pigs, and chest minecart duped).
[12:54:02] [Server thread/INFO]: Johnibur has made the advancement [We Need to Go Deeper]
[12:54:02] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 8038ms or 160 ticks behind
[12:54:02] [Client thread/INFO]: [CHAT] Johnibur has made the advancement [We Need to Go Deeper]
[12:54:02] [Client thread/INFO]: Loaded 32 advancements
[12:54:42] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 2094ms or 41 ticks behind
[12:55:05] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 7759ms or 155 ticks behind
[12:56:29] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 2029ms or 40 ticks behind
[13:01:38] [Client thread/WARN]: Unable to play empty soundEvent: minecraft:entity.tropical_fish.ambient
[13:01:55] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 8784ms or 175 ticks behind
[13:02:08] [Server thread/WARN]: Keeping entity minecraft:item that already exists with UUID 03514db8-902e-4053-96ff-a3d75b1fa444
[13:02:08] [Server thread/WARN]: Keeping entity minecraft:item that already exists with UUID 8d4677c9-b12a-4cb2-aee6-1490289eb051
[13:03:31] [Server thread/WARN]: Keeping entity minecraft:pig that already exists with UUID 410ab49d-de21-46b4-ab81-d601838a771e
[13:03:31] [Server thread/WARN]: Keeping entity minecraft:pig that already exists with UUID 3022c877-46ad-454a-9e20-789a3cace821
[13:03:31] [Server thread/WARN]: Keeping entity minecraft:pig that already exists with UUID c926c8aa-c1dc-42ba-8f64-55963dc877c4
[13:03:31] [Server thread/WARN]: Keeping entity minecraft:pig that already exists with UUID 228a4bee-009a-4797-acb7-029aa36e4e78
[13:03:31] [Server thread/WARN]: Keeping entity minecraft:chest_minecart that already exists with UUID 40c22265-232c-4844-836b-500454292710
[13:03:50] [Server thread/WARN]: Can't keep up! Is the server overloaded? Running 8971ms or 179 ticks behind
Makzevu
I believe this is caused by MC-138871.
Johnibur
Update Notice This is actually not related to Nether travelling. This duplication can be reproduced by randomly tping to an area where the chunks are not currently loaded. Also this duping never occurs when the chunks are constantly loaded. Thus using /forceload temporary solves this issue, although forceloading brings other issues by itself like mobs spawning being rarer and mobs actually still loaded on these areas but not despawing even if no players is nearby.
So the issue appears to be related to chunks loading. Might be linked to MC-138114.
Johnibur
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Cover me.
Johnibur
I think you should close this issue as duplicating MC-138871, because there is no way of getting this dupe exclusively only with Nether portals, especially if the chunks are already loaded on both sides.
michael