As always, it depends ... A fully equipped and completely enabled radar station with Mode-S will detect any inconsistency and there are. I remember a time where a workshop did Mode-S upgrades and only put in the aircraft ID and not the Mode-S, which resulted in a number of planes with the same 24-bit signature. They figured it out quite fast and resolved the issue, but I guess there are still a lot duplicate numbers around. If call sign, or flight number is put in the device it should be no problem as long as there are not too many signals around and pilots start selecting specific signals only.