My apologies if this has been discussed, but I find it annoying to have to mouse over the list of completed buildings on a planet to see if I've built a specific building. If I've built a building, shouldn't it not be displayed in the list of available buildings for that planet? Are there examples where you would want to build more than one example of a building? If so, should the building in the list be color-coded or have a "built quantity" number displayed to indicate this?
Presently there's nothing stopping you from building 800 copies of the same building on a planet.
Really what we need is for the buildable items list to have another toggle, which sets it to one of two modes:
1) All buildings and ships that your empire can produce (ie. the "available ones") are listed. Double-clicking available buildings or ships that can't be produced on the currently selected planet would do nothing.
2) Only buildings and ships that can be produced on the currently selected planet are listed. The others don't appear.
You'd set the toggle to mode 2. What we'd then need to do is adjust the location conditions of all (or most of) the buildings in the game so that they can't be built on a planet that already has one of those buildings.
Of course, doing that isn't quite so simple, as we'd have to deal with being able to enqueue more than one copy of a building at the same place, as technically if a building is just enqueued at particular location, it doesn't actually exist there yet, so another copy could be enqueued, even if it had a location condition saying it can't be built when there's another one present.
Eventually one of the buildings will finish, and mid-update, checks will need o be done to make sure every building on the queue can still be finished, and hasn't been made invalid by previously-finished buildings on the queue. Otherwise, even if you had a check each *turn* whether a building's location condition has become invalid since it was enqueued, if two of the building were enqueued the same turn, and got the same funding, or just happened to both finish being produced on the same turn, you could still end up with two copies since both were completed before the other was created in the universe.
This also has the potential to make order of empire turn processing important, as if one empire's buildings influenced where another empire's buildings could be produced, the results could depend on the order of stuff being taken of the various empires' queues.
There's also the issue of what to do with buildings or ships on queues that were valid to add when they were added, but which subsequently become invalid or not buildable at the location where they are being built. Do they just disappear from the queue, losing all PP that went towards them, or do they just hang? If the latter, a careful player could leave a nearly-complete backup copy of a certain building that's limited to one per empire, with the second one at a different location, but unable to complete due to the existance of the first. If the first is destroyed, the second immediately becomes a valid building, and can finish the next turn. That's possibly not what we'd want to be happening.
(I'm assuming there will be a variety of building location conditions created, related to resources or other buildings or other interesting factors... Making that possible is mainly why buildings have location conditions... (which determine where they can be built, if that wasn't clear)).