带有mysql的node.js中的ER_DUP_ENTRY

Question

使用以下代码时出现错误ER_DUP_ENTRY：

async.forEachOfSeries(classes, function(Myurl, m, eachDone) {
  var TheName= MyNames[m];
  var ThisThing=Things[m];

     async.waterfall([
      function(next) {
        connection.query(
          'SELECT * FROM mydatabase WHERE MyUrl=? LIMIT 1',
          [ Myurl ],
          next
          );
      },
        function(results,fields, next) {
          if (results.length !== 0) {
          console.log("Already Exists!");
          return next();
        }
          console.log("New Thing!");
          request(...)
          //Some other calculations...I.e. request(...)
          //And then:
          var post = {...}
          var query=connection.query('Insert INTO mydatabase Set ?', post, next);

    };
  });
 }
], eachDone);

}, function(err) {
  if (err) throw err; 

有关如何防止错误发生的任何想法？如果next中已有记录，如何正确跳至mydatabase？ 最诚挚的问候