You don't double flare in the aircraft industry because they use AN fittings only.
AN flares are 37 degrees and usually involve a tube sleeve to reinforce the tube.
Automotive stuff uses the double flare method because it's the most cost effective.they also use a 45 degree flare wich really thins and weakens the tube at the flare, hence the double flare. Why do they use the 45 degree flare ???? I would bet it had something to do with the tooling that was available way back when...