Might seem like this doesn't need to be said... but just in case...
If you enter two domain names that resolve to the same IP address, only the first one will appear. The second one is just ignored. I have a lot of domains on the same server, so I found this out and realized pretty quickly what was going on. But if you're going to ping places you don't own, you might not know what shares the same server.
A suggestion might be a pop-up that says something like "This IP address is already being tracked as xyz.com"
