apt-get upgrade fails with "adduser: The user `messagebus' already exists. Exiting."

The problem is the user id the messagebus user has.

First check what this command outputs:

grep messagebus /etc/passwd /etc/group

It should show something like this:

/etc/passwd:messagebus:x:102:104::/var/run/dbus:/bin/false
/etc/group:messagebus:x:104:

Then take a look at /etc/adduser.conf and search for "FIRST_SYSTEM_UID=111".
As you can see the ID of messagebus is 102 / 104 and below the value of FIRST_SYSTEM_UID.
Simply change FIRST_SYSTEM_UID to 100 and save (as root/sudo).

Then the upgrade should run without any errors.

Reference: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=634475