In 1975, the Communist government of Czechoslovakia revoked Duka's authorization to work as a priest. From then until the regime collapsed in 1989, Duka worked as a designer at the Škoda factory in Plzeň. In the meantime, he worked in secret in the Order and was elected the province's Vicar Provincial and served from 1975 until 1987. From 1976 to 1981, he taught theology to seminarians at the Theological Faculty of Litoměřice. In 1979, he obtained a licentiate in theology at the Theological Faculty of St. John the Baptist in Warsaw, Poland.