[DirectorTsaarx]

Well, except for the starting planets. If you start with multiple planets, the game will put a spaceport, resupply depot, and spaceyard on each one, even if they're all in the same system.

BTW - there's no reason you can't put multiple resupply depots in a system (other than needing the facility space for resource generation, etc.); if I have colonies at opposite ends of a "crossroads" system, sometimes I'll do this. Get a few extra movement points out of those exploration ships by putting a resupply depot at the planet nearest each warp point...