What is the Czech film about a girl and her double. The double is in a choir?

The film you're referring to is likely "Singing in the Dark" (original title: "Černý Petr"). It tells the story of a girl named Milena, who discovers her double, a girl in a choir. As the narrative unfolds, themes of identity and self-discovery are explored through the contrasting lives of the two girls. The film combines elements of drama and fantasy, showcasing the emotional complexity of their relationship.