In addition to the causes mentioned by DaveReidUK, transponder swaps between aircraft cause duplications as well. When a transponder is installed from the maintenance workshop and it was installed on a different airframe before, then the 24 bit address needs to be adjusted the new airframe. If forgotten, this typically causes a duplicate address within one airline's fleet, with a high change of both aircraft being in the same airspace at the same time. To prevent these errors, most new transponders now have an external memory module that is fixed to the aircraft so that reprogramming is not needed after a transponder swap; the address remains with the aircraft.
Originally Posted by DaveReidUK
Notwithstanding ICAO and EASA's exhortations, there doesn't seem to be any organised international effort to deal with the problem effectively. Unless anyone knows differently.
Eurocontrol addresses Mode-S anomalies in Europe, including duplicate 24 bit addresses. If their Mode-S monitoring cell is aware of a duplicate address they will try and find the operator and ask for correction of the address.