Most major orchestras insist on exclusivity clauses in the contracts of “superstar” soloists, so as to insure that ticket sales aren’t diluted. But such clauses rarely need to be enforced, since booking agents are well aware of the rules and make sure that their clients aren’t double-booked in a single city. But this week in Minneapolis, the Minnesota Orchestra and the Schubert Club each announced their new seasons, both featuring appearances by soprano Deborah Voigt, despite an exclusivity clause in the orchestra’s contract with the singer. Both groups agree that the mistake belongs to Voigt’s manager, but the orchestra is enforcing its exclusivity anyway, forcing the Schubert Club to cancel Voigt’s appearance, even though most of the club’s brochures had already been printed.