It will depend, in part, on the networking devices in use. A MAC address is supposed to be guaranteed globally unique. (Lots of assumptions made here.) Most likely, the switch would flip/flop back and forth between the two ports every 15s (the minimum time most switches appear to require before updating a forwarding database entry.).
I have no idea what you mean by, "What actually would happen underground?"
What the bad guy generally wants to do instead, is win the ARP race. This is done, usually, with lots of gratuitous ARP broadcasts and, simply, trying to answer the client's ARP request first.
As far as documentation, the attack is called ARP spoofing. If anything, read up about dug song's dsniff tool.
[Updated to address the comment.]
No, setting a static arp entry on the client would not protect from or prevent someone using macchanger as you described. If an attacker on your LAN were to
use macchanger as you described, it would create for a situation where two stations on your LAN have the same MAC address.
However, if you have a managed switch - meaning, a switch you can log in to and
configure. It almost certainly will support static forwarding entries. If you
were to configure the gateway's mac address statically on the gateway's switch
port, you would prevent an attacker from impersonating the gateway on your network. This is not the same as using arp -s.