RB
The same requirement has been present in the Eurocontrol Mode S Functional Specification since the first issue (1996)
https://www.eurocontrol.int/sites/de...n-20050509.pdf
(If I've failed to post the link correctly just Google "Eurocontrol Mode S specification")
" The Plot Assignor Function shall track all the aircraft, including aircraft with duplicated addresses and shall maintain the Roll Call list"
"The multiple target processing shall discriminate between false and real, nonunique addressed Mode S targets. The latter shall be flagged in the ASTERIX data item I048/030 Warning Error/Conditions bit 16 "Duplicated or Illegal Mode S Aircraft Address”
So any Mode S radar that claims complience to this spec will have these features and I'd be surprised if there are any around that don't.
RR