IPs and domains are not the same thing... i would suspect that someone else, a visitor maybe, is submitting... if not a (helpful?) visitor, maybe a competitor?
FWIW: i ran into this some years back... i think with altavista but don't remember for sure... if it was altavista, it was right about the time that they switched from being altavista.digital.com to altavista.com... now that i think about it, i'm sure it was AV... FWIW: they are also the *only* SE i've ever submitted my site to... all the others have just dropped by of their own volition :)
Ankheg - thankfully not! I have my own rang of IPs
WKitty - What I thought might be happening is other webmasters using the same shared/virtual server as myself might be submitting sites. As these would have the same (or similar) IPs that's what was causing the problem.