Seen in Uprev stage of all -paladin, e.g.
https://luci-logdog.appspot.com/logs/chromeos/buildbucket/cr-buildbucket.appspot.com/8925903228778573616/+/steps/Uprev/0/stdout

02:31:42: INFO: RunCommand: /b/swarming/w/ir/cache/cbuild/repository/chromite/bin/cros_mark_as_stable commit --all '--boards=atlas' --buildroot /b/swarming/w/ir/cache/cbuild/repository --overlay-type both in /b/swarming/w/ir/cache/cbuild/repository
02:32:03: ERROR: Found multiple stable ebuild versions in /b/swarming/w/ir/cache/cbuild/repository/src/third_party/chromiumos-overlay/sys-kernel/chromeos-kernel-4_14:
    sys-kernel/chromeos-kernel-4_14-4.14.89
    sys-kernel/chromeos-kernel-4_14-4.14.90
cros_mark_as_stable: Unhandled exception:

cd ~/chromiumos/src/third_party/chromiumos-overlay/sys-kernel/chromeos-kernel-4_14:
-rw-r--r-- 1 drinkcat primarygroup 1022 Dec 22 11:15 chromeos-kernel-4_14-4.14.89-r628.ebuild
-rw-r--r-- 1 drinkcat primarygroup 1022 Dec 25 14:03 chromeos-kernel-4_14-4.14.89-r634.ebuild
-rw-r--r-- 1 drinkcat primarygroup  901 May 14  2018 chromeos-kernel-4_14-9999.ebuild
drwxr-xr-x 2 drinkcat primarygroup 4096 May 14  2018 files
-rw-r--r-- 1 drinkcat primarygroup  244 May 14  2018 metadata.xml

For some reason there is a stale chromeos-kernel-4_14-4.14.89-r628.ebuild, added by this commit:

commit ed76fcfffeeb08df7233bc7895c8a62f759fb59b
Author: chrome-bot <chrome-bot@chromium.org>
Date:   Thu Dec 20 18:17:03 2018 -0800

    Automatic: master-lakitu-release - Updating to a new version number from 11439.0.0
    
    Change-Id: I0608bb9a857a65a578b6c07b6042128994fb5500