It isn’t unique, but fairly rare (see https://en.wikipedia.org/wiki/Portage)

I guess that’s because “similar parts of the world” are fairly rare. Requirements are that the shortcut is faster, safer, or cheaper (that Wikipedia page mentions an example where it was done to avoid taxes) than the detour over water, the land journey too steep (you’ll need beaches either side), and there’s a good reason to be at either side of the journey. I think that means you need population centers at either side.