I'd like the evolution deniers to come explain this.
Yes, it only takes two successive reproductions for the resistance mutation to be successful. And pretty soon it's spreading all over the land mass. Billions and billions of chances for the right mutation to have occurred since the resistant crops and Roundup spraying combination was introduced. Roundup takes care of all the competition in the gene pool pretty efficiently!
I just pray that the evolution deniers that couldn't forsee this don't conclude that an engineered virus is the best way to dispatch weeds next. Yeah, those never evolve and cross-over to species. Say corn and wheat? Oh, they'll just "patch that" with new virus-resistant corn and wheat? Sure. Because corporate profits should surely trump biodiversity in crops.
Perhaps a little legislation and regulation to make sure we don't make the planet die?