Basically, if you have a laggy enough server, the server stops keeping track of the players on it, and it results in duplicate players that can be killed, allowing for the duplication of items.
Also, sometimes players on the server do not show up under tab.
Related issues
Attachments
Comments
migrated
[media][media]
migrated
Caused by lag. I don't think Mojang can fix that. :s.
migrated
They should try and prevent the item duplication at least