While I was playing on my server I died from a Skeleton. After I picked up all my items I left and immediately closed a server. I opened a server again and saw this image : My items in inventory and the same items on the floor. Then I tried to dupe Diamonds the same way, and I did it. (No screenshots provided)

This happens if the dropped items were picked up before the server saves an information that you picked up all these items. (Probably)

This is very overpowered if you know when an Admin will restart the server.

I'm really sorry if there's any other people that created an Issue about the same problem. (I couldn't found atleast someone)

I use Server jar from : https://www.minecraft.net/en-us/article/minecraft-java-edition-1-15-2