It’s not that it bothers me but makes it impossible to operate the layout properly. If there are multiple cars with the same road name and number, how is one to know which one to pick up and were it goes. Then the real nightmare if they get into the same train. My heavens man, you could spot reefer #350121 at the packing plant thinking it was a clean empty, and they open it to find rotting strawberries that were supposed to have gone to the produce distribution center two towns away. That would be a very angry produce dealer, he might start shipping via trucks!
Combination of reporting mark and number must be unique.