If you want to show the machine id:

$ hostnamectl
 Static hostname: raspberrypi
       Icon name: computer
      Machine ID: 4f2201a9819d818f2bc928cc5ce05dca
         Boot ID: 2b213624e25384e3d2625bad5ba60ce2e
Operating System: Raspbian GNU/Linux 9 (stretch)
          Kernel: Linux 4.14.79-v7+
    Architecture: arm

Ways to get a hosts unique identifier:

$ cat /etc/machine-id

Debian should have another file which might be a symlink to the previous one:

$ cat /var/lib/dbus/machine-id

You can also use dbus-uuidgen:

$ dbus-uuidgen --get

You can use dbus-uuidgen to create a new UUID:

$ dbus-uuidgen
4f2201a9819d818f2bc928cc5ce05dca

From the man dbus-uuidgen:

D-Bus UUID has no relationship to RFC 4122 and does not generate UUIDs compatible with that spec.

To recreate the file rm -f /etc/machine-id and then issue:

$ dbus-uuidgen --ensure=/etc/machine-id

In Debian, also might need to do:

$ rm /var/lib/dbus/machine-id

And re-create it:

$ dbus-uuidgen --ensure

First boot

systemd-firstboot can be used to initialize basic system settings on or before the first boot-up of a system.

The service is started if ConditionFirstBoot=yes is satisfied. This essentially means that /etc is empty, see systemd.unit for details.

The following settings may be set up:

• system locale, more specifically the two locale variables LANG= and LC_MESSAGES

• system keyboard map

• system time zone

• system host name

• machine ID of the system

• root user's password

Recommended way to clone:

• clone the disk
• mount the cloned root partition somewhere (e.g. /mnt);
• initialize the machine id: systemd-firstboot --root=/mnt --setup-machine-id