There is no structure defined for the section names in the classic zipl.conf file. Thus the conversion script can't make any conclusions about machine-ids or such. I am going to open an upstream PR to document the limit of a single "rescue" entry into the man page and will close this as a NOTABUG for s390utils.

The issue not only happens with Rescue entries but also duplicated entries, e.g. you have an entry to boot the kernel "normally" and another entry to boot with "debug options":

/etc/zipl.conf:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
[defaultboot]
defaultauto
prompt=1
timeout=5
default=RHEL-Upgrade-Initramfs
target=/boot
[RHEL-Upgrade-Initramfs]
        image=/boot/vmlinuz-upgrade.s390x
        parameters="root=/dev/mapper/rootvg-root crashkernel=auto cio_ignore=all,!condev rd.lvm.lv=rootvg/root rd.dasd=0.0.0100 LANG=en_US.UTF-8 enforcing=0 rd.plymouth=0 plymouth.enable=0"
        ramdisk=/boot/initramfs-upgrade.s390x.img
[3.10.0-1160.36.2.el7.s390x]
        image=/boot/vmlinuz-3.10.0-1160.36.2.el7.s390x
        parameters="root=/dev/mapper/rootvg-root crashkernel=auto cio_ignore=all,!condev rd.lvm.lv=rootvg/root rd.dasd=0.0.0100 LANG=en_US.UTF-8"
        ramdisk=/boot/initramfs-3.10.0-1160.36.2.el7.s390x.img
[3.10.0-1160.36.2.el7.s390x_with_debugging]
        image=/boot/vmlinuz-3.10.0-1160.36.2.el7.s390x
        parameters="root=/dev/mapper/rootvg-root crashkernel=auto cio_ignore=all,!condev rd.lvm.lv=rootvg/root rd.dasd=0.0.0100 LANG=en_US.UTF-8 systemd.log_level=debug systemd.log_target=kmsg"
        ramdisk=/boot/initramfs-3.10.0-1160.36.2.el7.s390x.img
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------

Execute the conversion script:
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------
$ zipl-switch-to-blscfg --bls-directory=/tmp/bls
BLS file /tmp/bls/93f56661c34a47d4b5fd516d87aae9e0-3.10.0-1160.36.2.el7.s390x.conf already exists
-------- 8< ---------------- 8< ---------------- 8< ---------------- 8< --------


Increasing priority/severity to 2 because systems getting upgraded using Leapp end up restarting on RHEL7 kernel but RHEL8 root.

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