Yes, but subsequent factories still get the same throughput bonus. I tend to just continue stacking them because 1 steel mill in a state with 100 already present is in effect 1.5 steel mills, but in another state with none it'd just be back to 1.0.1 state with 120 steel mills produces same amount of steel, as 2 states with 60 steel mills. Once you hit that 51 mark, economies of scale don't stack up anymore.