What happens if there is a device already connected to a wifi network, and subsequently another device tries to join the wifi network using the exact same MAC address as the original?
A MAC address is supposed to be unique. If someone sabotages that then two devices on the network will claim to be the same unique NIC. That simply does not work.
Just how it breaks depends on the card and possibly on the driver.
Imagine two people in the same building. Both claiming to be employee number 1.
The left employee #1 shouts 'may I have some coffee?'
Someone fetches coffee and shouts 'Coffee for employee #1 is ready'.
That person now gets two replies, one 'thanks!' and 'Hey, I did not order coffee'.
And just which of the Employee #1's answers first might depend on how busy that person is, adding some more chaos to it.
So briefly: things break.
There isn't one answer and this depends on the switches that you use in your infrastructure.
In most cases, both computers will connect to the network and think they are working fine... however, you will see random failures and most likely not see the reason why.
In most Cisco switches, they rapidly flush the old MAC address the moment there is a packet from a different MAC and as such you may be able to do a few activities, but, the majority of network operations will fail.
In most other switches, they remember MAC addresses for around 3 minutes and you will see periods of 0 connectivity.
In all cases - it is highly likely that you will not be aware that the problem is that there is a MAC conflict.