Chinooker the reason for your frustration (absent some purely technical trouble like failure to update a database etc.) is an anomaly in what's called the "business rules" in IT speak, to whit, there should be a business rule that says that the aircraft can't be in two places at once, nor destined for two different destinations at once.
Of course the best part is that when this apparent computer anomaly is brought to the attention of the IT people, some very well spoken consultants in very nice suits and pastel ties will tell BA (or BAA) firstly that its their fault because they specified the business rules, and that fixing this "undocumented feature" of the system will cost a minimum of 750,000 pounds and take a minimum of six months (to allow for testing of course) to fix.
Of course that's if it doesn't require a complete redesign of the system.
The beauty of delivering large complex computer systems to companies and Governments is that the business rules will change faster than the software people will deliver (they like it that way), so with a bit of nifty footwork, there are always a string of changes necessary to accommodate changed work practices and the software firm is on the gravy train for life.
I've worked on both sides of one of these types of deals and I have the scars to prove it.