Logging out and in again fixes the issue, but I don't understand what is happening...
It seems that when the other user is still "signed in", it happens once but not twice after logging in with the qwerty account.
I have trouble with this, because at one point the computer was explicitely set up to qwerty, yet it was typing in azerty.
I'm a little confused and I can't find the old windows 7 config panel.
It's almost a bug.