I don't really spy on people - but when someone starts making trouble (esp a 'new' member) I do usually check to see if the IP matches someone else - and sometimes it does.
I will say though, a lot of complaints I get accuse the wrong people of alt identities. "Mr. X is really Ms. Y, and keeps bothering me..." But I'll check and find that, in all likelyhood, Mr. X is actually Ms. Z and that Ms. Z is having an affair with Mr. T, but what he doesn't know is that he isn't the father of Baby Z (and they all have to go on a daytime talk-show to do a paternity test to find out).