for h in $SERVER_LIST; do ssh $h "uptime"; done

The authenticity of host 'blah.blah.blah (10.10.10.10)' can't be established.
RSA key fingerprint is a4:d9:a4:d9:a4:d9a4:d9:a4:d9a4:d9a4:d9a4:d9a4:d9a4:d9.
Are you sure you want to continue connecting (yes/no)?

for h in $SERVER_LIST; do yes | ssh $h "uptime"; done

ssh -oStrictHostKeyChecking=no $h uptime

Host somehost
    Hostname 10.0.0.1
    StrictHostKeyChecking no

$ ssh -oStrictHostKeyChecking=no somehost uptime
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
@    WARNING: REMOTE HOST IDENTIFICATION HAS CHANGED!     @
@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@
IT IS POSSIBLE THAT SOMEONE IS DOING SOMETHING NASTY!
Someone could be eavesdropping on you right now (man-in-the-middle attack)!
It is also possible that a host key has just been changed.
The fingerprint for the RSA key sent by the remote host is
31:6f:2a:d5:76:c3:1e:74:f7:73:2f:96:16:12:e0:d8.
Please contact your system administrator.
Add correct host key in /home/peter/.ssh/known_hosts to get rid of this message.
Offending RSA key in /home/peter/.ssh/known_hosts:24
  remove with: ssh-keygen -f "/home/peter/.ssh/known_hosts" -R 10.0.0.1
Password authentication is disabled to avoid man-in-the-middle attacks.
Keyboard-interactive authentication is disabled to avoid man-in-the-middle attacks.
ash: uptime: not found

ssh-keyscan -H <ip-address> >> ~/.ssh/known_hosts
ssh-keyscan -H <hostname> >> ~/.ssh/known_hosts

ssh-keygen -R <ip-address>
ssh-keygen -R <hostname>

for h in $SERVER_LIST; do
    ip=$(dig +search +short $h)
    ssh-keygen -R $h
    ssh-keygen -R $ip
    ssh-keyscan -H $ip >> ~/.ssh/known_hosts
    ssh-keyscan -H $h >> ~/.ssh/known_hosts
done

ssh-keyscan  <newhost> >> ~/.ssh/known_hosts

# cat LIST_OF_SERVERS
server1
server2
...
server5
10.11.0.2
...
10.11.0.10

# for hh in `cat LIST_OF_SERVERS`; do ssh-keyscan ${hh} >> ~/.ssh/known_hosts ; done