That would be extremely difficult - if you bind accounts to IP's, you will eliminate all those that use proxy servers/ip sharing, not to mention dial up problems. You can't really verify that a person has not travelled from home to office etc. Unless your willing to get some real personal data and verify each one via the telephone/in person, I suspect someone will be able to set up as many accounts as they desire. (Especially with free ISPs, free email etc.) You can take steps to prevent users from using accounts on hotmail, yahoo and the like, but for many these have become primary email accounts and you could encounter problems with that also. I could be wrong, but at this point it would be very difficult to come up with a system that could verify this with the data most people are willing to supply.
