Yepp, that was it. I've had a similar bug once...I enabled all the VeteranX-stuff, without changing it...ingame, I suddenly had two Nighthawks...the normal one, and an aircraft one.
Why? 'cause unmodified, the British have VeteranAircraft=SHAD...
Yet, this brings up an interesting idea: With this bug, would it be possible to work around the 100-Unit-Bug?
Like, code a unit, set it as an AircraftType, but make it a VeteranUnit for everybody? The only question would be if the Air-To-Air-combat-thing gets overriden...