Whenever I do a fresh install of Debian etch and run the updates, whenever I restart a computer, I get a list of 4 different versions of Debian to run:

Debian GNU/Linux, kernal 2.6.18-6-k7
Debian GNU/Linux, kernal 2.6.18-6-k7 (single-user mode)
Debian GNU/Linux, kernal 2.6.18-5-k7
Debian GNU/Linux, kernal 2.6.18-5-k7 (single-user mode)

Is it normal for Debian to append to this list whenever an update is run?

Those are not different versions of Debian, they are only different kernel versions. Debian adds these when it updates to ensure that you can boot using an old kernel if for some reason the new one is broken or in some way incompatible with your hardware.

Yes, it is always a good idea to have multiple kernels in case your new kernel does not work. The single user lines are just different options for the kernel which keep it in a low run level. This can be important on a production box if you are having a problem with a daemon or you think you have been compromised.