[J65nko]

Because I am consistently lazy I hardly have multiple entries for the same host. And it is not difficult, because the message explicitly mentions the line number:

Offending ECDSA key in /home/adriaan/.ssh/known_hosts:30

And finding the line with the eye ball method is impossible if you use HashKnownHosts in your .ssh/config.
From ssh_config(5):

     HashKnownHosts
             Indicates that ssh(1) should hash host names and addresses when
             they are added to ~/.ssh/known_hosts.  These hashed names may be
             used normally by ssh(1) and sshd(8), but they do not reveal
             identifying information should the file's contents be disclosed.
             The default is ``no''.  Note that existing names and addresses in
             known hosts files will not be converted automatically, but may be
             manually hashed using ssh-keygen(1).

So:

$ tail -1 .ssh/known_hosts

192.168.222.249 ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAIbmlzdHAyNTYAAABBBHKXuTGoeWZ1TwAN23cwZzGAfOqjw
H1PZTxV+nUppIPnby4P/r1WPYu7M0YmUz2YEKrP0NkNERK3r/Xqc6PpPbU=

$  ssh-keygen -H -f .ssh/known_hosts
.ssh/known_hosts updated.

Original contents retained as .ssh/known_hosts.old
WARNING: .ssh/known_hosts.old contains unhashed entries
Delete this file to ensure privacy of hostnames

$  tail -1 .ssh/known_hosts
|1|DsPj+qiYhGO06l1W5yJQ0+J0VyQ=|sgJdo5xHyq3ncwsD8JLffrMOzEo= ecdsa-sha2-nistp256 AAAAE2VjZHNhLXNoYTItbmlzdHAyNTYAAAAI
bmlzdHAyNTYAAABBBHKXuTGoeWZ1TwAN23cwZzGAfOqjwH1PZTxV+nUppIPnby4P/r1WPYu7M0YmUz2YEKrP0NkNERK3r/Xqc6PpPbU=

As you can see the IP address 192.168.222.249 has now been hashed into |1|DsPj+qiYhGO06l1W5yJQ0+J0VyQ=|sgJdo5xHyq3ncwsD8J LffrMOzEo=
Impossible to find with the eye ball method