When a player joins the game with a second client (using the same account of course), while he still is in the game with the first client, the new player get logged in before the other player is kicked, Just as in this log snippet:

[22:40:37] [Server thread/INFO]: Player joined the game
[...]
[22:41:51] [Server thread/INFO]: Player joined the game
[...]
[22:41:51] [Server thread/INFO]: Player lost connection:
[...]
[22:42:09] [Server thread/INFO]: Player left the game

As you see the first client got kicked immediately after the second one joined the game. But the player ist loaded as it was saved the last time, not as the player that was just kicked. This can be abused to duplicate any Item or rideable entity.
e.g. the player can put stuff in a chest and then immediatly join using the second client. Then he has his stuff duplicated in the invetory AND in the chest. Or he can dismount a full packed donkey. Joining the game with the second client now loads the player riding a donkey again, thus duplicating the donkey including the donkey inventory.
In rare circumstance the first player stays around for some time giving the player the opportunity to kill 'himself' also leading to item duplication.
This bug also seems to confuse achievements and statistics. An achievemnt gained in the first clients session isn't saved and will be reset when the second client replaces the first. Similar things happen for statistics.