It's rare, but sometimes they do hit each other:

https://www.reddit.com/r/SelfDrivingCars/comments/1mdl5zn/tw...

https://www.reddit.com/r/waymo/comments/1pggtpu/two_waymos_m...

Lidar critics like to pretend that anti-collision is not a well-studied branch of Computer Science and telecoms. Wifi, Ethernet and cellphones all work well simultaneously, despite participants all sharing the same physical medium.