I have found the same problem, dating back nearly two years ago. Out of the 10 computers in our office (G5 Pros and mini's), 4 of them have had this Address Book problem. The com.apple.AddressBook.plist file becomes locked, then the Apple Computer Inc. entry in the Address Book replicates itself about once per minute. When the replication happens, a process called AddressBookSync pops up (have to watch the Activity Monitor) and quits after the new entry is added.
What's really odd is that even after I toss the preference file (though all you really have to do is unlock the file to stop the duplication), it always happens again a number of weeks later. I have yet to figure out what triggers this. As a band-aid, I wrote an AppleScript that checks every 10 minutes to see if that pref file is locked and, if it is, it will unlock it.
Anyway, I'm glad to hear we're not the only ones with this problem. Now if only someone could figure out what's causing it!