As DHU says Mode S Interrogators allow for the fact that two (or more) aircraft may have the same 24 bit ICAO address, it was quite common in the early days, not so now. Problems only arise when close in both azimuth and range. There is an indication in the radar message that gets sent off to the ATC center that an aircraft has a Duplicated Address, not sure what happens at the other end though. Is there any indication?
Reflection processing uses a number of factors before flagging a response as false (e.g. height, range, Aircraft Id etc.) and older Mode A/C systems survived without surpressing half the code 7000's buzzing around.
RR