The IP address might be in use, the MAC address wouldn't. In most corporate environments, Port Security is enabled, meaning that port would immediately shut down upon seeing the wrong MAC address.

Even if there were a duplicate IP on the network, it's a minor inconvenience at most, provided that it's not a domain controller or something.

You (the admin) notice a dupicate IP. You telnet/SSH/serial to the switch to see what's up. You find the two ports, one of which has the wrong MAC. Shut down the port and whoop the guy's ass.