am trying to update a column in the database, from a list :  command = new SqlCommand("update Login_Users set Password=@a  where UserName !='" + null + "'", Db);
                Db.Open();
                
                  

                for (int i = 0; i < list.Count; i++)
                {
                   list[i] = Encrypt(list[i]);
    
                 
                    command.Parameters.AddWithValue("@a",list[i]);
                    
                  
                    int a = command.ExecuteNonQuery();
                }


but i get this error variable name already been declared
