Every NIC card has a unique MAC address. As for your second question you will create a resource conflict if that scenario occurs. Each PC needs a unique IP address on the network whether it's statically configured or via DHCP which will automatically assign IP addresses via a server.
I hope this answers your question.
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MAC addresses have two components...the vendor code and the unique ID to that NIC. If you intentionally give two computers the same MAC address, you are "spoofing" the MAC address of at least one of those NICs. The technique is more of a malicious use and relates to security...I wouldn't worry too much about it for Network+ because I doubt they cover it. Essentially it's to capture traffic destined for another device at layer 2, or to defeat MAC filtering on a network appliance.
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I hope this answers your question.
*Bachelor's of Science: Information Technology - Security, Master's of Science: Information Technology - Management
Matthew 6:33 - "Seek the Kingdom of God above all else, and live righteously, and he will give you everything you need."
Certs/Business Licenses In Progress: AWS Solutions Architect, Series 6, Series 63