Comment # 13 on bug 1046709 from Stefan Dirsch

This sounds like a separate HOME partition, which you're reusing for each
installation. But then it doesn't make much sense to create an initial user
with the same name, which already exists on the separate HOME partition. It
will be overwritten. Still you may not be able to login with a user from the
separate HOME partition, if the system radically changed. But then you need to
look into the ~/.xsession-errors file to figure out, what's going wrong.

We don't claim to migrate user data for fresh installations, do we? Why not
using an update instead?