From what I can tell with the MRT PTX you get one 'unique' number on your transponder which is taken from a defunct AMB transponder - it's possible that there is a clone of it out there somewhere - then there are 20 other numbers in reserve which are the same 20 all MRTs have (also taken from 20 other txs) - so even if you had a heat of 21 drivers all with the MRTs and all of the 'unique numbers' clashed (think we're talking winning euromillions every week for a year odds here!) there would still be enough numbers to go round - that's as I understand it
