Currently “double breasted” is moderately out of style in western countries, meaning only a handful of “well dressed” men wear double breasted suits (e.g. the Prince of Wales, the Emperor of Japan – but not the Presidents of the United States or the Russian Federation). This changes frequently. As late as the 1980s they were definitely “in” on Wall Street and the “brown shoe law firms.”
Double-breasted blazers are still worn, and they are often part of full dress military uniforms (particularly of the navy). Heavy winter coats are more likely than not to be double-breasted. Note that this paragraph refers exclusivly to goyim. While we don’t copy them, we are influenced by them, albeit very slowly. By the time “double breasted” goes out of style among Yidden, it will probably be coming back into style among goyim.
In general, double-breasted is fancier and more formal, and Yidden tend to be very conservative. Among those of us wearing various versions of “frock” coats (Kapotes, Beckeshes, Rekels, etc.), the norm is double-breasted (intersting, in the situation where goyim still wear long coats, single breasted with a vest is the norm).