When is Fajr prayer called Fajr and when is it called Duha?

The Fajr prayer is never called the Duha prayer. Rather, the Duha prayer is a voluntary one, and its time begins after the sun has risen and ascended by the measure of a spear or two spears (approximately fifteen minutes after sunrise), and its time continues until just before the sun's zenith (approximately fifteen minutes or a little more before the Dhuhr prayer time begins).