Duplicate profiles on one player

When playing on minecraft with my partner we have realised that he has 2 different people for the same account so one day he’ll be a playing and get items and leaves them in his inventory the next time we can play it’ll put him as another profile so he can have nothing. This has been happening over a few months now and is difficult when building things as if he leaves items in his hot bar her can then temporarily loose them until the game puts him as the other player which also leads to him loosing items from the other player. This doesn’t happen every time on the game maybe one every 2 weeks he’ll switch and we’ll loose things until it switches him again