The micro:bit has a register mapped into it's memory called the FICR (Factory Information Configuration Register) that stores information programmed in at manufacturing time. One of the items programmed in is a unique* serial number. 

You can use this to identify the micro:bit in your program. Here's how to show the serial number on the display




from microbit import *
import machine

def get_serial_number(type=hex):
    NRF_FICR_BASE = 0x10000000
    DEVICEID_INDEX = 25 # deviceid[1]
    return type(machine.mem32[NRF_FICR_BASE + (DEVICEID_INDEX*4)]& 0xFFFFFFFF)

# Scroll the serial number on the LED display

Further Information

Also see How do I find the name of my micro:bit?

NRF51 SDK header file with register map

* The serial is actually a 64-bit factory generated pseudo-random number etched into each processor. Only 32 bits are used here though (as that makes it consistent for all the micro:bit higher level languages to deal with), and still enables it to be unique for most practical applications. To put this into context, you'd need about 100,000 to 250,000 micro:bits in one place to expect a duplication.

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