So i was playing on a server and fell into a hole with a hopper at the botttom. While i was still in the air i did /kill and then did /back hoping it would negate fall damage. What happened is that my items fell ito the hopper and i picked them up at the same time, but they were still in the hopper. I'm not sure if it's bug with a plugin or with Papermc, but if it's a vanilla dupe it would require extremely precise timing. I was unable to reproduce it, but this is something that would be good to investigate