It appears the same code is used for at least two different things: Unlocking the encrypted partition and logging in the main user. Since it is the same, it only has to be entered once at boot.
EDIT: Customer care says that two dialogs showing up is actually the default. So I might be wrong here.
If, however, at some point you had added another user (even if you deleted it in the meantime), the system will ask you separately for decrypting the partition and logging-in a user.
This resembles a desktop/server machine.
Done differently, it could instead log in the main user as before, and the main user could then switch users as he hands the device to the unprivileged user.
Yesterday I created a second user and apart from this causing the main user (me) to loose all data, I am now too suffering the double request - so actually I have to enter three codes, if I count the SIM PIN. A reflash would probably get rid of the problem, but I’m first looking for a specific solution.