Oftentimes the server can get spammed with warnings along the lines of:
[11:54:24] [Server thread/WARN]: Tried to add entity Sheep with pending removal and duplicate UUID d361162c-e5ef-41e8-8df9-741688e73c1a
[11:54:24] [Server thread/WARN]: Tried to add entity Villager with pending removal and duplicate UUID 44a5ac63-1215-4667-bed3-bdcde0055696
It appears that the root cause of these warnings is a delay of a tick (or maybe more) between entity removal, during which time a required chunk may be reloaded.
I have reproduced this in 14w49b Vanilla by flying around in creative at normal speed, but for a faster reproduction, the following may be helpful:
/give @p minecraft:golden_boots 1 0 {AttributeModifiers:[{AttributeName:"generic.movementSpeed",Name:"generic.movementSpeed",Amount:10,Operation:1,UUIDLeast:894654,UUIDMost:2872}],display:{Name:"Speed Boots",Lore:["Put on for a 10x Speed boost!"]}}.
It is likely this issue also affects 1.8.8 but is simply unnoticed as the warnings have only been added in the current snapshot series.
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Comments
md_5
migrated
but I have not yet reproduced this behaviour in Vanilla.
Then it appears this ticket is... well, you know the word.
md_5
No,
The issue mentioned in this ticket has been reproduced in Vanilla 15w49b.
The corollary mentioned in the comment has not.
[Mod] Neko
Can confirm in 15w50a.
migrated
and WAI per Searge's comment on MC-88922:
These log messages are "just" warnings that a potential problem with duplicate UUIDs was detected and prevented. It's not a problem, the code I wrote to fix the associated bug should work. I just left the messages in for now, so that I can see how often my code actually prevents it from happening, this will be changed eventually.
TL;DR: it's not a bug, just some useful debug logging
As a slightly related addendum, it appears that sometimes these duplicate entities can be saved across several chunks, but I have not yet reproduced this behaviour in Vanilla.