IMEI is the unique number identification of a cell / mobile phone. All mobile phones have a unique number - therefore you would not find two with the same number.

To check your IMEI number, key in "*#06#" (star hash zero six hash) on your Nokia mobile phone's keypad just as you would when you dial a phone number to make a call.

More detailed info. This only applies for phones after year 2004.
Format of IMEI is just like show below:
AA-BBBBBB-CCCCCC-D
AA-BBBBBB = Type Allocation Code (TAC)
CCCCCC = Serial sequence
D = Luhn check digit. (or zero if no checks)

From AA: you can know the Reporting Board - example 35 is BABT (British Approval Board for Telecommunications).