I've only played one game since I noticed this, but I've found a way to get multiple power plants on your planets. I assume that you shouldn't normally be able to do this.

First: You research until the second power plant tech, Anti-matter.
Second: You select the old tab, the one where you can build structures made obsolete by newer tech. You should find that the previous power plant structure, fussion, is available. In fact, you can build multiple fussion power plants.
Third: You research the final power plant tech, Quantum. This will trigger all of the fussion power plants to upgrade to Quantum.

As you might understand, the magnitude of possible production can get to be overwhelming.

Can anyone else confirm that this happens to them.

P.S. I don't think you need to finish building those structures for them to get upgraded.