In the past having name collisions would simply be a terrible convention, nowadays we have regulations that prevent it from happening with new names (same for housing).
High Streets can also be overtaken by events. Edinburgh's High Street hasn't been the most important street for anyone other than tourists for a few hundred years.
SE20 7QR, SE25 6EP, E13 0AP, E15 2LR, W5 5DB, W3 6LJ, NW10 4SJ, N14 6BW
For a map see . As you can see, plenty there are several roads the Royal Mail considers to be called High Street, London!