multiple keys in .ssh/known_hosts

I have an ssh tunnel from my home gateway to my machine at work. ie If
I ssh to a certain port on my home gateway from my home desktop I get my
work machine's ssh server. I must do this because my work machine has a
private address behind a firewall. The problem is that the
..ssh/known_hosts file on my home desktop has the public key for my
gateway in it and when I connect to the ssh tunnel my work machine's ssh
key gets sent over and my desktop tries to associate that key with my
gateway in the known_hosts file. This throws up an error and I have to
manually delete the key from the known_hosts file. Does anyone know a
way around this?

The tunnel command I use from my work machine is

ssh -f -C -Rport#:localhost:22 mygateway.com sleep 72h

Thanks,
TS