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It depends on the hex. The paragraph on timber, mine, and city hexes says, "Such hexes may be developed only once."
Industrial cities can be developed multiple times, up to their max on the chart.
For completeness, I'll note that you can't develop the start cities, farms, or Chicago at all. Players can't develop Detroit, but it develops itself.
This confirms what I was thinking. The question came up in regards to developing the industrial cities, and then I couldn't find the ruling on developing the other hexes only once. Thanks.