MILAN (Reuters) – Hong Kong-listed Italian luxury group Prada SpA said on Thursday it had bought a stake in the Tuscan tannery Superior SpA, tightening its grip on its supply chain, but noted it had not decided whether to list in Milan. A dual listing in Europe would help Prada widen its investor base, as some investment funds can only put money in European or U.S. stocks. “No decision has been taken,” on a secondary listing in Milan, Chief Executive Patrizio Bertelli said in an interview with Italian daily newspaper Il Corriere della Sera. In July, Prada’s Chairman Paolo Zannoni said a second…