This is a glitch involving the "open to lan" function in the game. A single player can open 2 windows of Minecraft then proceed to multiplayer with himself in Minecraft.
The problem is that of one account is logged in and the other isn't the offline in account will have the items and the xp from when the player last played offline. The player then can use the offline account's items and xp and give the product to the online account.
The offline account is refreshed with the same stats if the player closes the offline account window, saves and quits the online account window (with the items they want duplicated), closes said window, opens another minecraft window and plays the same world offline.
I believe this can be fixed by putting in coding that says two IP adresses that are identical log each other out of minecraft. Or simply making it so that you cannot detect lan worlds that are "local host".
A player could easily open to lan and allow cheats on for a world without cheats enabled but that doesn't provide as quick a method to duplicate items and experience they had as the previous method provides.
This method can be replicated in any gamemode that can open to lan.
My minecraft account is BobTheGodly and I don't cheat.