1) Log-in to the server
2) Teleport to your base
3) Have items you want to dupe ready in your inventory
4) Open a chest/enderchest
5) Place items into chest/enderchest
6) Quickly change Wi-Fi network to something else (DO NOT RE-LOG)
7) You will get booted, join the server ASAP.
8) Your items will be back on you & chest will have duplicates