It also adds a check into the "New Manual Build" page to ensure that users can't create a buildable for an object that already has one.

At least tentatively, I think this should be allowed. If you're testing build plans, it lets you basically create a copy of some buildable and muck around with it without messing up the "real" one. There probably should be a flag to distinguish the "real" one from synthetic copies, though. For example, I wouldn't want a build to go red and send a bunch of alerts out because I messed up when configuring a new plan or whatever.

The existing code looks up the buildable by the object's PHID when applying a plan, so we'd need to change the code to allow a buildable's PHID to be passed through (otherwise it will always pick the first buildable, and not necessarily the one that you applied the plan to).