Per AC43-13
"...In forming flares, cut the tube ends square, file smooth, remove all burrs and sharp edges, and thoroughly clean. The tubing is then flared using the correct 37-degree aviation flare forming tool for the size of tubing and type of fitting. A double flare is used on soft aluminum tubing 3/8-inch outside diameter and under, and a single flare on all other tubing..."
This is in the Hydraulic tubing section for low pressure applications.
I think most folks just use a single flare. I opted to buy the double flare tool, Master Cool brand, and have been happy with the results...It was the 37 degree kit no additional dies were required...