This is because you have used the same "Varname" for all questions. Please edit your questions and make sure that none of your questions have the same "Varname", UNLESS they are in separate categories.
In other words:
Category Mutui Ipotecari, per Acquisto e Ristrutturazione Immobili, question Tipo di tasso and Category Mutui Ipotecari, per Sostituzione, Surroga, Liquidita', question Tipo di tasso can have the same varname.
Two questions within the same category can not.