For an somewhat challenging introduction to the subject, Wikipedia has a good article on "MAC address". It stands for "medium access control", with "medium" meaning the network. "As typically represented, MAC addresses are recognizable as six groups of two hexadecimal digits, separated by hyphens, colons, or without a separator", and every "NIC (network interface controller)" in the world is meant to have a different MAC address. Data is sent on the local network from one NIC to another using the MAC address to select the destination NIC.
This causes trouble when a manufacturer actually releases NICs which have the same MAC address as each other. They are meant not to do that.
If you do have a duplicated card, then you may have to throw it away. "Many network interfaces, however, support changing their MAC addresses" - for instance with a software command while the host boots up or connects - so you could do that and set an acceptable MAC address, or even a random hexadecimal number as the address - I refer you to Wikipedia for reasons to do that. A genuinely random address is extremely unlikely to be a duplication.
I'm not certain but I think that if your local networking connects every host directly to a central device (hub), then the local network may have only your host and the hub talking to each other by MAC address anyway - or something like that - and then it wouldn't matter.
For a while, some software licensing and copyright protection depended on the software being used on a computer with one MAC address that you'd paid for. You could possibly override that by replacing the NIC and then setting its MAC address to the required number. Reasonably of course - PCs and network devices do fail and need to be replaced.