They're similar, but not exactly the same. Fast flite is comprised primarily of a material called spectra, D-97 of a material called dyneema. They are both high tech low stretch, high strength, small diameter strings. They are much more similar to each other than to dacron, which by comparison is larger diameter, less abrasion resistant and higher stretch.
Fastflite is the trademarked name for the first low stretch, high strength material on the market. There are lots of them now which are generically referred to as fast flite type strings. D-97 is among them.