GalCiv II only lets you create one orbital fleet manager per planet (Fine). However, I find that if I conquer an alien colony the computer may have done something dumb like build an orbital fleet manager on a research bonus tile. So if I replace the orbital fleet manager with another structure (e.g. research lab), then I won't be able to build another orbital fleet manager on that planet because they game still thinks I already built one and doesn't realise that it has been destroyed/replaced.