No 2 devices should ever have the same MAC they are hard coded by the manufacturer. Because Espressif has more than 1 set of MAC addresses for the ESP8266 ( I have both 5C/5E and 60/62) it is possible to have 2 devices running ESPEasy with the same
chipID. The chances are less than 1 in 281,474,976,710,656. ( feel free to check my math, I am not perfect

). I think you have better odds for winning the lottery, even the Italian lottery.
Where you may have a problem is your router may not be able to handle the number of IP addresses. A lot of carrier provided routers limit the number of connected devices, and really limit the number of manually assigned IP Addresses. If I remember correctly my current router(vodafone) is limited to 27 devices, and about 10 assigned addresses. Not enough, but the ESP devices do not need internet access so they can be on a separate network.