In reality, this does not happen that way. If a path already exists, you can pay to use the same duct (unless it's full) to install your own fibers. At least, it works this way in France.
It absolutely does happen that way in Germany. We had Fiber Company A rip up the entire city a year ago, and Fiber Company B ripped up the streets again just a few weeks ago.
ETA from my ISP to actually get any of those lines into my apartment is still 2028.
This is a coördination problem: the municipality can mandate that every company has to install a larger conduit so that (say) 3-6 (?) other fibres can be run.
So if one company is doing the east part of the town, and another company is doing the west, at some point they could leverage the infrastructure of each other.
I don't think so. German open access law says if you're in a "market dominating" position, you have to share access to what you have. I'm at least unaware of a law that allows forcing telcos to build more than they want. (And even if there were, it'd be balanced by them deciding to just not build at all.)
I'm glad it works that way there. In the Netherlands, I've never seen a fiber duct laid in residential areas, it seems just plain fiber cables are buried.
Worse, two companies are chasing each other, so you have the ground being opened, closed, and within half a year the other company does the same. Even worse, they let the fiber cables stick out of the ground until the house subscribes to them, so a lot of houses have 1 or 2 ugly orange cables sticking out of the ground for years near the front door.
ETA from my ISP to actually get any of those lines into my apartment is still 2028.