the ID duplication effects
Terminology is often a cause of confusion with Mode S. There are actually two identities:
The 24-bit address is allocated to an airframe and "should" be unique across the entire aircraft population. Occasionally errors occur but in principle it is unchanging and unique.
Mode S also supports Aircraft ID (sometimes called ACID). This is an eight-character alphanumeric that is set on the flight deck for every flight. Normally for a commercial flight this will be the flight number. Simplistically this should match directly with the number on the flight plan, thus removing the need for the old code/callsign correlation process on the ground. I say "simplistically" because it has been a technical and bureaucratic labour of many years turning that apparently simple mechanism into reality. Now when an aircraft is seen to be "squawking 1000" it is because the new direct matching of ACID to Flight Plan is in force, and the Mode A has been relegated to conspicuity only.
Needless to say, when a radar has found a duplicate 24-bit address it would be well advised to check that the Aircraft ID and the altitude are also identical before even beginning to consider this may be a "reflection".