My position and inventory are resetted after I re-join after changing username

I changed my username and now after that whenever I join my friend's server I am reset to the position and inventory as I had the first time after I changed the username.

Scenario:
I mine downwards, lava appears and I die. I lose all my diamonds tools etc etc. I respawn and then leave and the join the server again. Have regained all my tools and levels from the moment I joined after changing my username.

I also can put my inventory in a chest, leave and join, and I have duplicated the inventory.