Sure, they won't be exactly the same - but give it 20 generations and there won't be a significant difference. The people will largely be genetically the same,

Sure, they won't be exactly the same - but give it 20 generations and there won't be a significant difference. The people will largely be genetically the same, the population and its distribution will be largely the same - they'll fit into the same ecological niche, they'll be inventing the same things and polluting the same rivers. There is potential for the butterfly effect to do something - the right change to an unstable equilibrium certainly has the potential to trigger a cascade of events. In general though, such events are highly improbable - and even dramatically cascading events will have their impacts dampened across time and space.