There is nothing built into IIS to limit connections on a per ip basis to only allow one connection. It's all or nothing. IP's can be blocked or granted, but nothing on a per connection basis.. You could write code the store the ip address based on server variables, then refuse connection, but that is beyond IIS.
Steve Schofield
Windows Server MVP - IIS
http://weblogs.asp.net/steveschofield
http://www.IISLogs.comLog archival solution
Install, Configure, Forget