How to launch a SSH tunnel with upstart

I am not very used to sys admin and recently created an SSH tunnel between two servers (on Ubuntu 12.4) with the following command:

ssh -fNg -L 3307:127.0.0.1:3306 tunneluser@xx.xx.xx.xx

It worked, I also added this command to /etc/rc.local, and it successfully launch the SSH tunnel at startup. However, I tried to add my tunnel in a file in /etc/init/my_tunnel.conf, and it worked but launched nearly 12 tunnels at every startup! I used the following code:

start on (local-filesystems and net-device-up IFACE=eth0)stop on runlevel[016]respawnexec ssh -fNg -L 3307:127.0.0.1:3306 tunneluser@xx.xx.xx.xxexit 0

I spent hours on this and do not understand at all why and how this script is executed so many times. I also tried start on[2345], completely read upstart's manual but still the same. I am sure that I am missing something here.

If someone can help me. Thanks.