> what prevents collisions from two miners mining the same bitcoin at the same time? Absolutely nothing. There can, in fact, be two blocks at the same height.

> what prevents collisions from two miners mining the same bitcoin at the same time? Absolutely nothing. There can, in fact, be two blocks at the same height. This is a temporary fork. What happens is that miners mine the next block on top of one or the other at random, and the chain which gets the next block "wins", and everyone then mines on top of that. To make it clearer, at any time, the longest chain of blocks wins. There can be two chains of the same length at any point, but that doesn't last for long, as it's statistically incredibly unlikely that this will continue for more than a block or so. There's a page full of times that it's happened, here: https://blockchain.info/orphaned-blocks https://blockchain.info/orphaned-blocks