It tells you that you have already assigned a RSH key to that address. The new key is different to this and thus the "strict checking" disables the new session.
Go to your known_HOSTS file and delete it
on my mac it looks like this:
192.168.1.1 ssh-rsa AAA......
10.0.0.2 ssh-rsa AAAAB3NzaC1yc.......
10.0.0.3 ssh-rsa AA.......
10.0.0.20 ssh-rsa AAAAB3Nza......
Thanks Brokk. Funny, I hadn't used VI in around 12 years or so, but I was able to remember the editing commands and remove that line from the file.
(Mac e.g vi known_hosts, dd (deletes line in question), :wq (save and exit)) - Done!
I hope this helps someone else as it seems every time we reinstall another image on the Raspberry Pi, we might have this happen.
IAS